Answer
$E = \frac{\rho~r}{3~\epsilon_0}$
Work Step by Step
We can find the charge enclosed within a sphere of radius $r$:
$q = \rho~V$
$q = \rho~\frac{4}{3}\pi~r^3$
We can find the electric field at $P$:
$\epsilon_0~\Phi = q$
$(\epsilon_0)~(E)~(4\pi~r^2) = \rho~\frac{4}{3}\pi~r^3$
$\epsilon_0~E = \frac{\rho~r}{3}$
$E = \frac{\rho~r}{3~\epsilon_0}$
