Factoring can be tricky. For this problem, you have to start with the 6x^2. There are basically two ways you can get 6x^2; 6x times x and 2x times 3x. Then you look at the -20; There are a few more ways to get this; 1 times 20, 2 times 10, and 4 times 5. Since they gave you choices, you could try each out until you found one that worked. If you don't have choices, you just have to try different combinations until it works.