Minimizer Square

Let $X$ be a random variable with mean $\mu$ and variance $\sigma^2$, both finite. Find the constant $c$ that minimizes $f(c) = \mathbb{E}\left[ (X - c)^2 \right]$. Report the answer when $\mu = 4$ and $\sigma^2 = 9$. If it's not possible to find such a value of $c$, report $-1$.