I am trying to solve this problem but I am not able to do it.

$y=e^{rx}$ : For what values of $r$ does $y'' - 2y' - 3y = 0$.
I tried to differentiate $y$ with respect to $x$, but I get something like : $e^{rx}\cdot \frac{d}{dx}(rx)$, which does not seem to help me a lot.

How I am supposed to do this ?