OK, let's crack this. We're given 864 and our task is to multiply it by 31. Essentially, we'll be adding 864 to itself 31 times.
Step 1: 0 + 864 = 864
Step 2: 864 + 864 = 1728
Step 3: 1728 + 864 = 2592
Step 4: 2592 + 864 = 3456
Step 5: 3456 + 864 = 4320
Step 6: 4320 + 864 = 5184
Step 7: 5184 + 864 = 6048
Step 8: 6048 + 864 = 6912
Step 9: 6912 + 864 = 7776
Step 10: 7776 + 864 = 8640
Step 11: 8640 + 864 = 9504
Step 12: 9504 + 864 = 10368
Step 13: 10368 + 864 = 11232
Step 14: 11232 + 864 = 12096
Step 15: 12096 + 864 = 12960
Step 16: 12960 + 864 = 13824
Step 17: 13824 + 864 = 14688
Step 18: 14688 + 864 = 15552
Step 19: 15552 + 864 = 16416
Step 20: 16416 + 864 = 17280
Step 21: 17280 + 864 = 18144
Step 22: 18144 + 864 = 19008
Step 23: 19008 + 864 = 19872
Step 24: 19872 + 864 = 20736
Step 25: 20736 + 864 = 21600
Step 26: 21600 + 864 = 22464
Step 27: 22464 + 864 = 23328
Step 28: 23328 + 864 = 24192
Step 29: 24192 + 864 = 25056
Step 30: 25056 + 864 = 25920
Step 31: 25920 + 864 = 26784

So, 864*31 = 26784