y2+5y+6 over y2 -5y+6
Factorize numerator and denominator.
(y2 + 5y + 6) / (y2 - 5y + 6) = [((y + 2) (y + 3)] / [( y - 2 )(y - 3)].
Remember, never to divide by zero!
SO, exclude y = 2 or y =3 (inclusive or, that means either y = 2, y = 3 OR both) because those values make the denominator zero.
No further simplification seems possible.