A particular planet is spherically symmetric, but has a non-constant density given by $\rho \left( r \right) ={ \rho }_{ 0 }\left( 1-\frac {{ r}^{2} }{ {R}^{2} } \right)$, where $R$ is the radius of the planet.

What is the magnitude of the gravitational field at a distance of $3R$ from the center of the planet?