Trying to factor all trinomials using guess and check can be time consuming. If, after half a minute, you're still having trouble factoring a trinomial, the AC method is a step-by-step procedure that will get the job done. This material is covered on pages 231-236 in the Engaging Algebra text.
5.7 Factoring with the AC Method
After mastering these videos you'll be able to factor trinomials using the AC method.