To factorize the quadratic expression #x^2-9x+20#, one must find the two numbers that add together to give 9, and multiply together to give 20.

Those two numbers are 4 and 5, which factor into the expression #(x-4)(x-5)#. The signs are due to the fact that the first sign in the expression is a positive, which tells us that both signs must be the same, and the first sign is a negative, which means that, due to the positive sign in the expression, both signs are negative.

To do the opposite, expanding, one must multiply #x# by #x#, to give #x^2#, add together #4# and #5# to give #9#, and multiply them to give #20#.