Let's expand the left side of the equation first:

(x + 3)(x - 2) = x^2 + x(-2) + 3x - 6
(x - 3)(x + 2) = x^2 + x(2) - 3x - 6

Now, we can rewrite the original equation with the expansions:

(x^2 + x - 6) - (x^2 + 2x - 3x - 6) - 5 = 6x - y

Simplify the equation:

x - 6 - x^2 - 2x + 3x + 6 - 5 = 6x - y
x - x^2 - 2x + 3x - 5 = 6x - y
-x^2 + 2x - 5 = 6x - y

Rearrange terms to get the equation in the standard form:

-x^2 + 2x - 5 - 6x + y = 0
-x^2 - 4x - 5 + y = 0

Therefore, the simplified equation is -x^2 - 4x - y - 5 = 0.