OK, let's crack this. We're given 91 and our task is to multiply it by 39. Essentially, we'll be adding 91 to itself 39 times.
Step 1: 0 + 91 = 91
Step 2: 91 + 91 = 182
Step 3: 182 + 91 = 273
Step 4: 273 + 91 = 364
Step 5: 364 + 91 = 455
Step 6: 455 + 91 = 546
Step 7: 546 + 91 = 637
Step 8: 637 + 91 = 728
Step 9: 728 + 91 = 819
Step 10: 819 + 91 = 910
Step 11: 910 + 91 = 1001
Step 12: 1001 + 91 = 1092
Step 13: 1092 + 91 = 1183
Step 14: 1183 + 91 = 1274
Step 15: 1274 + 91 = 1365
Step 16: 1365 + 91 = 1456
Step 17: 1456 + 91 = 1547
Step 18: 1547 + 91 = 1638
Step 19: 1638 + 91 = 1729
Step 20: 1729 + 91 = 1820
Step 21: 1820 + 91 = 1911
Step 22: 1911 + 91 = 2002
Step 23: 2002 + 91 = 2093
Step 24: 2093 + 91 = 2184
Step 25: 2184 + 91 = 2275
Step 26: 2275 + 91 = 2366
Step 27: 2366 + 91 = 2457
Step 28: 2457 + 91 = 2548
Step 29: 2548 + 91 = 2639
Step 30: 2639 + 91 = 2730
Step 31: 2730 + 91 = 2821
Step 32: 2821 + 91 = 2912
Step 33: 2912 + 91 = 3003
Step 34: 3003 + 91 = 3094
Step 35: 3094 + 91 = 3185
Step 36: 3185 + 91 = 3276
Step 37: 3276 + 91 = 3367
Step 38: 3367 + 91 = 3458
Step 39: 3458 + 91 = 3549

So, 91*39 = 3549