I need help with defining the integral of a marginal pdf

Question

Given: $f(x,y)=1/\pi a^2,$ when $x^2+y^2\le a^2\,(a>0)$ In order to find the marginal pdfs, I have to set up the integral. I don't understand how to go about it as the expression is composed of constants only.

Answer 1

\begin{align} f_x(x) &= \int_{-\sqrt{a^2 -x^2}}^{\sqrt{a^2 -x^2}} f(x,y) \, dy \\ &= \left(\frac{y}{\pi a^2}\right)_{-\sqrt{a^2 -x^2}}^{\sqrt{a^2 -x^2}} \\ &= \frac{2\sqrt{a^2 -x^2}}{\pi a^2} \end{align}