The joint probability density function of $X$ and $Y$ is given by $$f(x,y)=c(y^2 - 36 x^2)e^{-y}, \ \ \ - \frac{y}{6} \le x \le \frac{y}{6}, \ \ 0 < y < \infty$$ Find $c$ and the expected value of $X$:
$c =$
$E(X) =$