[tex] \frac{ {(a + x)}^{2} - 9}{(a + x) + 3} [/tex]
[tex] \frac{ {(a + x)}^{2} - {3}^{2} }{(a + x) + 3} [/tex]
[tex] \frac{(a + x + 3)(a + x - 3)}{a + x + 3} [/tex]
= a + x - 3
Saludos
