OK, let's work through this together. We're starting with 136 and we're going to multiply it by 21, which means we add 136 to itself 21 times.
Step 1: 0 + 136 = 136
Step 2: 136 + 136 = 272
Step 3: 272 + 136 = 408
Step 4: 408 + 136 = 544
Step 5: 544 + 136 = 680
Step 6: 680 + 136 = 816
Step 7: 816 + 136 = 952
Step 8: 952 + 136 = 1088
Step 9: 1088 + 136 = 1224
Step 10: 1224 + 136 = 1360
Step 11: 1360 + 136 = 1496
Step 12: 1496 + 136 = 1632
Step 13: 1632 + 136 = 1768
Step 14: 1768 + 136 = 1904
Step 15: 1904 + 136 = 2040
Step 16: 2040 + 136 = 2176
Step 17: 2176 + 136 = 2312
Step 18: 2312 + 136 = 2448
Step 19: 2448 + 136 = 2584
Step 20: 2584 + 136 = 2720
Step 21: 2720 + 136 = 2856

So, 136*21 = 2856