OK, let's work through this together. We're starting with 680 and we're going to multiply it by 84, which means we add 680 to itself 84 times.
Step 1: 0 + 680 = 680
Step 2: 680 + 680 = 1360
Step 3: 1360 + 680 = 2040
Step 4: 2040 + 680 = 2720
Step 5: 2720 + 680 = 3400
Step 6: 3400 + 680 = 4080
Step 7: 4080 + 680 = 4760
Step 8: 4760 + 680 = 5440
Step 9: 5440 + 680 = 6120
Step 10: 6120 + 680 = 6800
Step 11: 6800 + 680 = 7480
Step 12: 7480 + 680 = 8160
Step 13: 8160 + 680 = 8840
Step 14: 8840 + 680 = 9520
Step 15: 9520 + 680 = 10200
Step 16: 10200 + 680 = 10880
Step 17: 10880 + 680 = 11560
Step 18: 11560 + 680 = 12240
Step 19: 12240 + 680 = 12920
Step 20: 12920 + 680 = 13600
Step 21: 13600 + 680 = 14280
Step 22: 14280 + 680 = 14960
Step 23: 14960 + 680 = 15640
Step 24: 15640 + 680 = 16320
Step 25: 16320 + 680 = 17000
Step 26: 17000 + 680 = 17680
Step 27: 17680 + 680 = 18360
Step 28: 18360 + 680 = 19040
Step 29: 19040 + 680 = 19720
Step 30: 19720 + 680 = 20400
Step 31: 20400 + 680 = 21080
Step 32: 21080 + 680 = 21760
Step 33: 21760 + 680 = 22440
Step 34: 22440 + 680 = 23120
Step 35: 23120 + 680 = 23800
Step 36: 23800 + 680 = 24480
Step 37: 24480 + 680 = 25160
Step 38: 25160 + 680 = 25840
Step 39: 25840 + 680 = 26520
Step 40: 26520 + 680 = 27200
Step 41: 27200 + 680 = 27880
Step 42: 27880 + 680 = 28560
Step 43: 28560 + 680 = 29240
Step 44: 29240 + 680 = 29920
Step 45: 29920 + 680 = 30600
Step 46: 30600 + 680 = 31280
Step 47: 31280 + 680 = 31960
Step 48: 31960 + 680 = 32640
Step 49: 32640 + 680 = 33320
Step 50: 33320 + 680 = 34000
Step 51: 34000 + 680 = 34680
Step 52: 34680 + 680 = 35360
Step 53: 35360 + 680 = 36040
Step 54: 36040 + 680 = 36720
Step 55: 36720 + 680 = 37400
Step 56: 37400 + 680 = 38080
Step 57: 38080 + 680 = 38760
Step 58: 38760 + 680 = 39440
Step 59: 39440 + 680 = 40120
Step 60: 40120 + 680 = 40800
Step 61: 40800 + 680 = 41480
Step 62: 41480 + 680 = 42160
Step 63: 42160 + 680 = 42840
Step 64: 42840 + 680 = 43520
Step 65: 43520 + 680 = 44200
Step 66: 44200 + 680 = 44880
Step 67: 44880 + 680 = 45560
Step 68: 45560 + 680 = 46240
Step 69: 46240 + 680 = 46920
Step 70: 46920 + 680 = 47600
Step 71: 47600 + 680 = 48280
Step 72: 48280 + 680 = 48960
Step 73: 48960 + 680 = 49640
Step 74: 49640 + 680 = 50320
Step 75: 50320 + 680 = 51000
Step 76: 51000 + 680 = 51680
Step 77: 51680 + 680 = 52360
Step 78: 52360 + 680 = 53040
Step 79: 53040 + 680 = 53720
Step 80: 53720 + 680 = 54400
Step 81: 54400 + 680 = 55080
Step 82: 55080 + 680 = 55760
Step 83: 55760 + 680 = 56440
Step 84: 56440 + 680 = 57120

So, 680*84 = 57120