OK, let's work through this together. We're starting with 850 and we're going to multiply it by 21, which means we add 850 to itself 21 times.
Step 1: 0 + 850 = 850
Step 2: 850 + 850 = 1700
Step 3: 1700 + 850 = 2550
Step 4: 2550 + 850 = 3400
Step 5: 3400 + 850 = 4250
Step 6: 4250 + 850 = 5100
Step 7: 5100 + 850 = 5950
Step 8: 5950 + 850 = 6800
Step 9: 6800 + 850 = 7650
Step 10: 7650 + 850 = 8500
Step 11: 8500 + 850 = 9350
Step 12: 9350 + 850 = 10200
Step 13: 10200 + 850 = 11050
Step 14: 11050 + 850 = 11900
Step 15: 11900 + 850 = 12750
Step 16: 12750 + 850 = 13600
Step 17: 13600 + 850 = 14450
Step 18: 14450 + 850 = 15300
Step 19: 15300 + 850 = 16150
Step 20: 16150 + 850 = 17000
Step 21: 17000 + 850 = 17850

So, 850*21 = 17850