First determine if a common monomial factor (Greatest Common Factor) exists. Factor trees may be used to find the GCF of difficult numbers. Be aware of opposites: Ex. (a-b) and (b-a), These two may be the same if we factor out-1 to any of the expressions. 3x - 12 = 3(x-4) x²y² - 3xy² = xy² (x-3) 6(x - y) + a(x - y) = (x - y) (6 + a) ​