First, you’d look at what can multiply together to make 20. You can multiply 10x2, 1x20, or 4x5 (or change them so each number is a negative) to make a positive 20. You then must look at which one of these can be added or subtracted together to create -9. Negative 4 and 5 can be subtracted to make -9, so you would factor them out, as well as the x^2, and get the answer (x-4)(x-5).
Answer: (x-4)(x-5)
