An Electrostatics PROBLEM

The field potential inside a charged ball depends only on the distance from its centre as $\phi = 5r^4 -6r^2$ , where $r$ is the distance from the centre. Then find the value of $r$ where the value of volume charge density $\rho(r)$ vanishes ( i.e becomes zero). Assume all the quantities are in S.I units and therefore enter your answer as the value of $r$ in metres.