I understand the basic idea of factorising, but I keep getting confused about which two numbers to pick. Can someone explain a reliable method and show how it works on x^2 + 7x + 12?
Look for two numbers that multiply to 12 and add to 7. Those numbers are 3 and 4, so x^2 + 7x + 12 becomes (x + 3)(x + 4). A reliable method is: first check the number term, list factor pairs, then test which pair gives the middle coefficient.