OK, let's work through this together. We're starting with 810 and we're going to multiply it by 86, which means we add 810 to itself 86 times.
Step 1: 0 + 810 = 810
Step 2: 810 + 810 = 1620
Step 3: 1620 + 810 = 2430
Step 4: 2430 + 810 = 3240
Step 5: 3240 + 810 = 4050
Step 6: 4050 + 810 = 4860
Step 7: 4860 + 810 = 5670
Step 8: 5670 + 810 = 6480
Step 9: 6480 + 810 = 7290
Step 10: 7290 + 810 = 8100
Step 11: 8100 + 810 = 8910
Step 12: 8910 + 810 = 9720
Step 13: 9720 + 810 = 10530
Step 14: 10530 + 810 = 11340
Step 15: 11340 + 810 = 12150
Step 16: 12150 + 810 = 12960
Step 17: 12960 + 810 = 13770
Step 18: 13770 + 810 = 14580
Step 19: 14580 + 810 = 15390
Step 20: 15390 + 810 = 16200
Step 21: 16200 + 810 = 17010
Step 22: 17010 + 810 = 17820
Step 23: 17820 + 810 = 18630
Step 24: 18630 + 810 = 19440
Step 25: 19440 + 810 = 20250
Step 26: 20250 + 810 = 21060
Step 27: 21060 + 810 = 21870
Step 28: 21870 + 810 = 22680
Step 29: 22680 + 810 = 23490
Step 30: 23490 + 810 = 24300
Step 31: 24300 + 810 = 25110
Step 32: 25110 + 810 = 25920
Step 33: 25920 + 810 = 26730
Step 34: 26730 + 810 = 27540
Step 35: 27540 + 810 = 28350
Step 36: 28350 + 810 = 29160
Step 37: 29160 + 810 = 29970
Step 38: 29970 + 810 = 30780
Step 39: 30780 + 810 = 31590
Step 40: 31590 + 810 = 32400
Step 41: 32400 + 810 = 33210
Step 42: 33210 + 810 = 34020
Step 43: 34020 + 810 = 34830
Step 44: 34830 + 810 = 35640
Step 45: 35640 + 810 = 36450
Step 46: 36450 + 810 = 37260
Step 47: 37260 + 810 = 38070
Step 48: 38070 + 810 = 38880
Step 49: 38880 + 810 = 39690
Step 50: 39690 + 810 = 40500
Step 51: 40500 + 810 = 41310
Step 52: 41310 + 810 = 42120
Step 53: 42120 + 810 = 42930
Step 54: 42930 + 810 = 43740
Step 55: 43740 + 810 = 44550
Step 56: 44550 + 810 = 45360
Step 57: 45360 + 810 = 46170
Step 58: 46170 + 810 = 46980
Step 59: 46980 + 810 = 47790
Step 60: 47790 + 810 = 48600
Step 61: 48600 + 810 = 49410
Step 62: 49410 + 810 = 50220
Step 63: 50220 + 810 = 51030
Step 64: 51030 + 810 = 51840
Step 65: 51840 + 810 = 52650
Step 66: 52650 + 810 = 53460
Step 67: 53460 + 810 = 54270
Step 68: 54270 + 810 = 55080
Step 69: 55080 + 810 = 55890
Step 70: 55890 + 810 = 56700
Step 71: 56700 + 810 = 57510
Step 72: 57510 + 810 = 58320
Step 73: 58320 + 810 = 59130
Step 74: 59130 + 810 = 59940
Step 75: 59940 + 810 = 60750
Step 76: 60750 + 810 = 61560
Step 77: 61560 + 810 = 62370
Step 78: 62370 + 810 = 63180
Step 79: 63180 + 810 = 63990
Step 80: 63990 + 810 = 64800
Step 81: 64800 + 810 = 65610
Step 82: 65610 + 810 = 66420
Step 83: 66420 + 810 = 67230
Step 84: 67230 + 810 = 68040
Step 85: 68040 + 810 = 68850
Step 86: 68850 + 810 = 69660

So, 810*86 = 69660