Hint: this is quadratic in $\displaystyle \sqrt[4]{x}$. to see this, let $\displaystyle y = \sqrt[4]{x}$, then we obtain
$\displaystyle y^2 - 3y - 40 = 0$
now solve this quadratic for $\displaystyle y$, and then you can find $\displaystyle x$
as always, check your solutions when finished