OK, let's work through this together. We're starting with 315 and we're going to multiply it by 12, which means we add 315 to itself 12 times.
Step 1: 0 + 315 = 315
Step 2: 315 + 315 = 630
Step 3: 630 + 315 = 945
Step 4: 945 + 315 = 1260
Step 5: 1260 + 315 = 1575
Step 6: 1575 + 315 = 1890
Step 7: 1890 + 315 = 2205
Step 8: 2205 + 315 = 2520
Step 9: 2520 + 315 = 2835
Step 10: 2835 + 315 = 3150
Step 11: 3150 + 315 = 3465
Step 12: 3465 + 315 = 3780

So, 315*12 = 3780