Let's first expand both sides of the equation:

(8-9x)x = -40 + (6-3x)(6+3x)
8x - 9x^2 = -40 + 36 - 9x^2

Now, let's combine like terms:

8x - 9x^2 = -4

Rearranging the terms, we get:

9x^2 - 8x - 4 = 0

Now, we can solve for x by using the quadratic formula:

x = (-(-8) ± √((-8)^2 - 4(9)(-4))) / 2(9)
x = (8 ± √(64 + 144)) / 18
x = (8 ± √208) / 18

Therefore, the solutions for x are:

x = (8 + √208) / 18 or x = (8 - √208) / 18