OK, let's work through this together. We're starting with 900 and we're going to multiply it by 7, which means we add 900 to itself 7 times.
Step 1: 0 + 900 = 900
Step 2: 900 + 900 = 1800
Step 3: 1800 + 900 = 2700
Step 4: 2700 + 900 = 3600
Step 5: 3600 + 900 = 4500
Step 6: 4500 + 900 = 5400
Step 7: 5400 + 900 = 6300

So, 900*7 = 6300