Alright, let's solve this math problem step by step. First, we have the number 876. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 21. So, we're going to take the number 876 and add it to itself 21 times.
Step 1: 0 + 876 = 876
Step 2: 876 + 876 = 1752
Step 3: 1752 + 876 = 2628
Step 4: 2628 + 876 = 3504
Step 5: 3504 + 876 = 4380
Step 6: 4380 + 876 = 5256
Step 7: 5256 + 876 = 6132
Step 8: 6132 + 876 = 7008
Step 9: 7008 + 876 = 7884
Step 10: 7884 + 876 = 8760
Step 11: 8760 + 876 = 9636
Step 12: 9636 + 876 = 10512
Step 13: 10512 + 876 = 11388
Step 14: 11388 + 876 = 12264
Step 15: 12264 + 876 = 13140
Step 16: 13140 + 876 = 14016
Step 17: 14016 + 876 = 14892
Step 18: 14892 + 876 = 15768
Step 19: 15768 + 876 = 16644
Step 20: 16644 + 876 = 17520
Step 21: 17520 + 876 = 18396

So, 876*21 = 18396