The factors of 12 are 1, 2, 3, 4, 6, and 12. The factors of 39 are 1, 3, 13, and 39. A GCF, or greatest common factor, is the largest (greatest) number that divides two numbers. From the lists, we see that this is 3. If we factor out the 3 from both 12 and 39 essentially undoing the distributive property, we are left with the product 3 * ( 4 + 13 ).
