How can i solve 9+21 as to factor the expression using the (GCF)

The GCF is 9 because all the numbers are divisible by 9

9/9 = 1 and 21/9 = 3

now we are left with 1+3, but we don't get rid of the GCF

You have to put the GCF in front of your new expression

You should now have 9 (1+3)