Sure thing! We've got 645 and we're gonna multiply it by 6. That's the same as adding 645 to itself 6 times.
Step 1: 0 + 645 = 645
Step 2: 645 + 645 = 1290
Step 3: 1290 + 645 = 1935
Step 4: 1935 + 645 = 2580
Step 5: 2580 + 645 = 3225
Step 6: 3225 + 645 = 3870

So, 645*6 = 3870