OK, let's work through this together. We're starting with 258 and we're going to multiply it by 21, which means we add 258 to itself 21 times.
Step 1: 0 + 258 = 258
Step 2: 258 + 258 = 516
Step 3: 516 + 258 = 774
Step 4: 774 + 258 = 1032
Step 5: 1032 + 258 = 1290
Step 6: 1290 + 258 = 1548
Step 7: 1548 + 258 = 1806
Step 8: 1806 + 258 = 2064
Step 9: 2064 + 258 = 2322
Step 10: 2322 + 258 = 2580
Step 11: 2580 + 258 = 2838
Step 12: 2838 + 258 = 3096
Step 13: 3096 + 258 = 3354
Step 14: 3354 + 258 = 3612
Step 15: 3612 + 258 = 3870
Step 16: 3870 + 258 = 4128
Step 17: 4128 + 258 = 4386
Step 18: 4386 + 258 = 4644
Step 19: 4644 + 258 = 4902
Step 20: 4902 + 258 = 5160
Step 21: 5160 + 258 = 5418

So, 258*21 = 5418