Foci Concentric Circles

Question

My approach: Using the foci formula $$c=\sqrt{a^2-b^2}$$. By plugging in $a=3$ and $b=2$ I obtain plus and minus $\sqrt{5}$. But there's 2 choices with a root 5 result. How do i know which one is which?

Answer 1

From the figure, $a$ is $3$ and $b$ is $2$. Assuming the center of the circles are at the origin, $c=\pm \sqrt{5}$. Where are you stuck?