Answer
$\{\pm\frac{5}{2},\pm5\}$
Work Step by Step
Step 1. Let $u=(x)^{-2}$, we have $625u^2-125u+4=0$
Step 2. Factor and solve: $(25u-4)(25u-1)=0$ thus $u=\frac{1}{25}, \frac{4}{25}$
Step 3. For $u=\frac{1}{25}$, we have $(x)^{-2}=\frac{1}{25}$ thus $x=\pm5$
Step 4. For $u=\frac{4}{25}$, we have $(x)^{-2}=\frac{4}{25}$ thus $x=\pm\frac{5}{2}$
Step 5. Solution set $\{\pm\frac{5}{2},\pm5\}$