When multiplying together two binomials, a useful strategy is to "FOIL" it -- first, outside, inside, last. That is to say, multiply together the two first terms, the two outside terms, the two inside terms, and the two last terms. This is just an easy way to remember to multiply both things on the left by both things on the right, so however you want to remember that works.
FOIL might be easier to understand with an example. Say you have to find the product of
For your problem, FOILed it is
FOIL comes from the distributive property, which states that
Going back to