To solve this, we need to first expand the brackets on the left side:

(х+8)(х+3) = х^2 + 3х + 8х + 24 = х^2 + 11х + 24

Now, we substitute this back into the original equation:

х^2 + 11х + 24 - 40 = 40х^2 + 440х + 960

Now, we simplify the equation:

х^2 + 11х - 16 = 40х^2 + 440х + 960

Rearranging terms, we get:

39х^2 + 429х + 976 = 0

Now, we can solve this quadratic equation using the quadratic formula:

х = (-429 ± √(429^2 - 439976))/(2*39)

х = (-429 ± √(183441 - 151104))/78

х = (-429 ± √32337)/78

Therefore, the solutions for x are:

x = (-429 + √32337)/78 and x = (-429 - √32337)/78