Solve for $\displaystyle m$

$\displaystyle 5m^4+24m^2-36=0\;m^2=y\Rightarrow 5y^2+24y-36=0$

Then i plugged it into the quadratic formula

$\displaystyle y=\frac{-24\pm\sqrt{24^2-(4)(5)(-36)}}{10}\Rightarrow y=\frac{-24\pm36}{10}\;y=\frac{6}{5}\;y=-6$

From there i did $\displaystyle m^2=\frac{6}{5}\Rightarrow m=\pm\sqrt\frac{6}{5}$

$\displaystyle m^2=-6$ Which gives an imaginary answer, so I really cant use that as answer right?

P.S. I used $\displaystyle m=\pm\sqrt\frac{6}{5}$ as my answer and I got marked off 1 point anyone know why?