OK, let's work through this together. We're starting with 672 and we're going to multiply it by 42, which means we add 672 to itself 42 times.
Step 1: 0 + 672 = 672
Step 2: 672 + 672 = 1344
Step 3: 1344 + 672 = 2016
Step 4: 2016 + 672 = 2688
Step 5: 2688 + 672 = 3360
Step 6: 3360 + 672 = 4032
Step 7: 4032 + 672 = 4704
Step 8: 4704 + 672 = 5376
Step 9: 5376 + 672 = 6048
Step 10: 6048 + 672 = 6720
Step 11: 6720 + 672 = 7392
Step 12: 7392 + 672 = 8064
Step 13: 8064 + 672 = 8736
Step 14: 8736 + 672 = 9408
Step 15: 9408 + 672 = 10080
Step 16: 10080 + 672 = 10752
Step 17: 10752 + 672 = 11424
Step 18: 11424 + 672 = 12096
Step 19: 12096 + 672 = 12768
Step 20: 12768 + 672 = 13440
Step 21: 13440 + 672 = 14112
Step 22: 14112 + 672 = 14784
Step 23: 14784 + 672 = 15456
Step 24: 15456 + 672 = 16128
Step 25: 16128 + 672 = 16800
Step 26: 16800 + 672 = 17472
Step 27: 17472 + 672 = 18144
Step 28: 18144 + 672 = 18816
Step 29: 18816 + 672 = 19488
Step 30: 19488 + 672 = 20160
Step 31: 20160 + 672 = 20832
Step 32: 20832 + 672 = 21504
Step 33: 21504 + 672 = 22176
Step 34: 22176 + 672 = 22848
Step 35: 22848 + 672 = 23520
Step 36: 23520 + 672 = 24192
Step 37: 24192 + 672 = 24864
Step 38: 24864 + 672 = 25536
Step 39: 25536 + 672 = 26208
Step 40: 26208 + 672 = 26880
Step 41: 26880 + 672 = 27552
Step 42: 27552 + 672 = 28224

So, 672*42 = 28224