my question is

2x^2-5x+7-x^2+3x-8

x^2-2x-15 x^2-2x-15

the common factor is (x-5)(x+3) but do still multiply the numerator with all of the denominator. or do i collect like terms. If i collect like terms i got

3x^4+2x+1. and i cant factor that out