First, we want to find the GCF of 15 and 21, which is 3. Now we do 15 divided by 3 and 21 divided by 3, which gives us 5 and 7. To rewrite the expression, we take the GCF and put it on the outside of the parenthesis. Inside the parenthesis, we put 5 + 7. So it'll look like this:
