For: $\displaystyle z^4 + 7z^2 +2$

how can I use the quadratic formula to find the complex roots?

I tried substituting$\displaystyle u = z^2$, but when I use the quadratic formula I get real answers and don't know where to substitute $\displaystyle u$ back in.

I also tried thinking of the values for a b and c as z^2, 7 and 2, but I cam across difficulties when trying to simplify the contents of the square root.