Let's roll up our sleeves and solve this. We have 84 and we're going to multiply it by 45, essentially adding 84 to itself 45 times.
Step 1: 0 + 84 = 84
Step 2: 84 + 84 = 168
Step 3: 168 + 84 = 252
Step 4: 252 + 84 = 336
Step 5: 336 + 84 = 420
Step 6: 420 + 84 = 504
Step 7: 504 + 84 = 588
Step 8: 588 + 84 = 672
Step 9: 672 + 84 = 756
Step 10: 756 + 84 = 840
Step 11: 840 + 84 = 924
Step 12: 924 + 84 = 1008
Step 13: 1008 + 84 = 1092
Step 14: 1092 + 84 = 1176
Step 15: 1176 + 84 = 1260
Step 16: 1260 + 84 = 1344
Step 17: 1344 + 84 = 1428
Step 18: 1428 + 84 = 1512
Step 19: 1512 + 84 = 1596
Step 20: 1596 + 84 = 1680
Step 21: 1680 + 84 = 1764
Step 22: 1764 + 84 = 1848
Step 23: 1848 + 84 = 1932
Step 24: 1932 + 84 = 2016
Step 25: 2016 + 84 = 2100
Step 26: 2100 + 84 = 2184
Step 27: 2184 + 84 = 2268
Step 28: 2268 + 84 = 2352
Step 29: 2352 + 84 = 2436
Step 30: 2436 + 84 = 2520
Step 31: 2520 + 84 = 2604
Step 32: 2604 + 84 = 2688
Step 33: 2688 + 84 = 2772
Step 34: 2772 + 84 = 2856
Step 35: 2856 + 84 = 2940
Step 36: 2940 + 84 = 3024
Step 37: 3024 + 84 = 3108
Step 38: 3108 + 84 = 3192
Step 39: 3192 + 84 = 3276
Step 40: 3276 + 84 = 3360
Step 41: 3360 + 84 = 3444
Step 42: 3444 + 84 = 3528
Step 43: 3528 + 84 = 3612
Step 44: 3612 + 84 = 3696
Step 45: 3696 + 84 = 3780

So, 84*45 = 3780