Question 2

If $\mathbf{a} + \mathbf{b} + \mathbf{c} = 0$ and $|\mathbf{a}| = 3$, $|\mathbf{b}| = 5$ and $|\mathbf{c}| = 7$. Find the angle between $\mathbf{a}$ and $\mathbf{b}$.