Let's roll up our sleeves and solve this. We have 168 and we're going to multiply it by 21, essentially adding 168 to itself 21 times.
Step 1: 0 + 168 = 168
Step 2: 168 + 168 = 336
Step 3: 336 + 168 = 504
Step 4: 504 + 168 = 672
Step 5: 672 + 168 = 840
Step 6: 840 + 168 = 1008
Step 7: 1008 + 168 = 1176
Step 8: 1176 + 168 = 1344
Step 9: 1344 + 168 = 1512
Step 10: 1512 + 168 = 1680
Step 11: 1680 + 168 = 1848
Step 12: 1848 + 168 = 2016
Step 13: 2016 + 168 = 2184
Step 14: 2184 + 168 = 2352
Step 15: 2352 + 168 = 2520
Step 16: 2520 + 168 = 2688
Step 17: 2688 + 168 = 2856
Step 18: 2856 + 168 = 3024
Step 19: 3024 + 168 = 3192
Step 20: 3192 + 168 = 3360
Step 21: 3360 + 168 = 3528

So, 168*21 = 3528