Alright, let's solve this math problem step by step. First, we have the number 840. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 144. So, we're going to take the number 840 and add it to itself 144 times.
Step 1: 0 + 840 = 840
Step 2: 840 + 840 = 1680
Step 3: 1680 + 840 = 2520
Step 4: 2520 + 840 = 3360
Step 5: 3360 + 840 = 4200
Step 6: 4200 + 840 = 5040
Step 7: 5040 + 840 = 5880
Step 8: 5880 + 840 = 6720
Step 9: 6720 + 840 = 7560
Step 10: 7560 + 840 = 8400
Step 11: 8400 + 840 = 9240
Step 12: 9240 + 840 = 10080
Step 13: 10080 + 840 = 10920
Step 14: 10920 + 840 = 11760
Step 15: 11760 + 840 = 12600
Step 16: 12600 + 840 = 13440
Step 17: 13440 + 840 = 14280
Step 18: 14280 + 840 = 15120
Step 19: 15120 + 840 = 15960
Step 20: 15960 + 840 = 16800
Step 21: 16800 + 840 = 17640
Step 22: 17640 + 840 = 18480
Step 23: 18480 + 840 = 19320
Step 24: 19320 + 840 = 20160
Step 25: 20160 + 840 = 21000
Step 26: 21000 + 840 = 21840
Step 27: 21840 + 840 = 22680
Step 28: 22680 + 840 = 23520
Step 29: 23520 + 840 = 24360
Step 30: 24360 + 840 = 25200
Step 31: 25200 + 840 = 26040
Step 32: 26040 + 840 = 26880
Step 33: 26880 + 840 = 27720
Step 34: 27720 + 840 = 28560
Step 35: 28560 + 840 = 29400
Step 36: 29400 + 840 = 30240
Step 37: 30240 + 840 = 31080
Step 38: 31080 + 840 = 31920
Step 39: 31920 + 840 = 32760
Step 40: 32760 + 840 = 33600
Step 41: 33600 + 840 = 34440
Step 42: 34440 + 840 = 35280
Step 43: 35280 + 840 = 36120
Step 44: 36120 + 840 = 36960
Step 45: 36960 + 840 = 37800
Step 46: 37800 + 840 = 38640
Step 47: 38640 + 840 = 39480
Step 48: 39480 + 840 = 40320
Step 49: 40320 + 840 = 41160
Step 50: 41160 + 840 = 42000
Step 51: 42000 + 840 = 42840
Step 52: 42840 + 840 = 43680
Step 53: 43680 + 840 = 44520
Step 54: 44520 + 840 = 45360
Step 55: 45360 + 840 = 46200
Step 56: 46200 + 840 = 47040
Step 57: 47040 + 840 = 47880
Step 58: 47880 + 840 = 48720
Step 59: 48720 + 840 = 49560
Step 60: 49560 + 840 = 50400
Step 61: 50400 + 840 = 51240
Step 62: 51240 + 840 = 52080
Step 63: 52080 + 840 = 52920
Step 64: 52920 + 840 = 53760
Step 65: 53760 + 840 = 54600
Step 66: 54600 + 840 = 55440
Step 67: 55440 + 840 = 56280
Step 68: 56280 + 840 = 57120
Step 69: 57120 + 840 = 57960
Step 70: 57960 + 840 = 58800
Step 71: 58800 + 840 = 59640
Step 72: 59640 + 840 = 60480
Step 73: 60480 + 840 = 61320
Step 74: 61320 + 840 = 62160
Step 75: 62160 + 840 = 63000
Step 76: 63000 + 840 = 63840
Step 77: 63840 + 840 = 64680
Step 78: 64680 + 840 = 65520
Step 79: 65520 + 840 = 66360
Step 80: 66360 + 840 = 67200
Step 81: 67200 + 840 = 68040
Step 82: 68040 + 840 = 68880
Step 83: 68880 + 840 = 69720
Step 84: 69720 + 840 = 70560
Step 85: 70560 + 840 = 71400
Step 86: 71400 + 840 = 72240
Step 87: 72240 + 840 = 73080
Step 88: 73080 + 840 = 73920
Step 89: 73920 + 840 = 74760
Step 90: 74760 + 840 = 75600
Step 91: 75600 + 840 = 76440
Step 92: 76440 + 840 = 77280
Step 93: 77280 + 840 = 78120
Step 94: 78120 + 840 = 78960
Step 95: 78960 + 840 = 79800
Step 96: 79800 + 840 = 80640
Step 97: 80640 + 840 = 81480
Step 98: 81480 + 840 = 82320
Step 99: 82320 + 840 = 83160
Step 100: 83160 + 840 = 84000
Step 101: 84000 + 840 = 84840
Step 102: 84840 + 840 = 85680
Step 103: 85680 + 840 = 86520
Step 104: 86520 + 840 = 87360
Step 105: 87360 + 840 = 88200
Step 106: 88200 + 840 = 89040
Step 107: 89040 + 840 = 89880
Step 108: 89880 + 840 = 90720
Step 109: 90720 + 840 = 91560
Step 110: 91560 + 840 = 92400
Step 111: 92400 + 840 = 93240
Step 112: 93240 + 840 = 94080
Step 113: 94080 + 840 = 94920
Step 114: 94920 + 840 = 95760
Step 115: 95760 + 840 = 96600
Step 116: 96600 + 840 = 97440
Step 117: 97440 + 840 = 98280
Step 118: 98280 + 840 = 99120
Step 119: 99120 + 840 = 99960
Step 120: 99960 + 840 = 100800
Step 121: 100800 + 840 = 101640
Step 122: 101640 + 840 = 102480
Step 123: 102480 + 840 = 103320
Step 124: 103320 + 840 = 104160
Step 125: 104160 + 840 = 105000
Step 126: 105000 + 840 = 105840
Step 127: 105840 + 840 = 106680
Step 128: 106680 + 840 = 107520
Step 129: 107520 + 840 = 108360
Step 130: 108360 + 840 = 109200
Step 131: 109200 + 840 = 110040
Step 132: 110040 + 840 = 110880
Step 133: 110880 + 840 = 111720
Step 134: 111720 + 840 = 112560
Step 135: 112560 + 840 = 113400
Step 136: 113400 + 840 = 114240
Step 137: 114240 + 840 = 115080
Step 138: 115080 + 840 = 115920
Step 139: 115920 + 840 = 116760
Step 140: 116760 + 840 = 117600
Step 141: 117600 + 840 = 118440
Step 142: 118440 + 840 = 119280
Step 143: 119280 + 840 = 120120
Step 144: 120120 + 840 = 120960

So, 840*144 = 120960