U substitution uses the variable "u" as a replacement for another variable. In this case, we will substitute in u for x^2. Since u=x^2 we can now rewrite the equation. 9u^2 -2u -7. This is factored to (u-1)(9u+7). Now put x^2 back in for u. (x^2 -1)(9x^2 +7). Factor again. (x-1)(x+1)(9x^2 + 7). Since 9x^2 + 7 doesn't factor, we don't bother with it any more. The answer is x=1 or x=-1.