OK, let's work through this together. We're starting with 644 and we're going to multiply it by 3, which means we add 644 to itself 3 times.
Step 1: 0 + 644 = 644
Step 2: 644 + 644 = 1288
Step 3: 1288 + 644 = 1932

So, 644*3 = 1932