OK, let's work through this together. We're starting with 648 and we're going to multiply it by 18, which means we add 648 to itself 18 times.
Step 1: 0 + 648 = 648
Step 2: 648 + 648 = 1296
Step 3: 1296 + 648 = 1944
Step 4: 1944 + 648 = 2592
Step 5: 2592 + 648 = 3240
Step 6: 3240 + 648 = 3888
Step 7: 3888 + 648 = 4536
Step 8: 4536 + 648 = 5184
Step 9: 5184 + 648 = 5832
Step 10: 5832 + 648 = 6480
Step 11: 6480 + 648 = 7128
Step 12: 7128 + 648 = 7776
Step 13: 7776 + 648 = 8424
Step 14: 8424 + 648 = 9072
Step 15: 9072 + 648 = 9720
Step 16: 9720 + 648 = 10368
Step 17: 10368 + 648 = 11016
Step 18: 11016 + 648 = 11664

So, 648*18 = 11664