Three forces act simultaneously on a space satellite in such a way that the satellite remains in…

Three forces act simultaneously on a space satellite in such a way that the satellite remains in equilibrium. The forces are represented by three vectors $\vec{p}$, $\vec{q}$ and $\vec{r}$, such that $\vec{p} + \vec{q} + \vec{r} = \vec{0}$, $|\vec{p}| = 3$, $|\vec{q}| = 5$ and $|\vec{r}| = 7$. Find the angle between the forces $\vec{p}$ and $\vec{q}$.