A=lw
x^2-9=(x-3)(l)
divide both sides by (x-3)
x^2-9/(x-3)=(x-3)(l)/(x-3)
x^2-9 is equal to (x-3)(x+3)
(x+3)(x-3)/(x-3)=l --> the (x-3)/(x-3) cancels out to become one which leaves the l, and that's what's going to happen to the left side of the = sign now
(x+3)=l ANSWER
