Factorise and solve x^2 - 8x + 15 = 0.

15 can be rewritten as 3*5, so our factorisation must be in the form (x ? 3)(x ? 5). To determine the signs inside the brackets we need to find a way to make -8 from 3 and 5. This is done with -8=-5-3, so we can factorise our equation with x^2-8x+15=(x-3)(x-5)=0. Therefore our solutions must be x=3 and x=5.