Simplify:
(X^2 - 9)(7x + 21) / (x- 3)(x^2 + 6x + 9) =
everything factors nicely. Things are already partially factored giving you clues as what the other factors will be
(X^2 - 9)(7x + 21) / (x- 3)(x^2 + 6x + 9)
= (x+3)(x-3)(7)(x+3) / ( (x-3)(x+3)(x+3)
= 7 , x ≠ ± 3