Simple expression:$ a - a^{-1}$ = ...

Question

I got stuck with one simple expression, I hope get some help with it:

If $a-\frac{1}{a}=\frac{3\sqrt7}{7}$, so $a^4+\frac{1}{a^4}=$

Answer 1

Square each side of your equation, move constant terms to the right hand side, and square again. The answer will come out clearly.