Question

(X+2)(x-3)=0. Then x=9

Answer 1

i mean substitute

Answer 2

(X+2)(X-3)

(9+2)(9-3)
(11)(6)
11x6

Answer 3

$$
(x+2)(x-3)=0\\
@x=9,\\
(9+2)(9-3)=11\times6=66
$$
Since this is not zero, x=9 is NOT a solution.

Hope this helps. :-)