Vector Geometry

Let $\mathbf{a}$ and $\mathbf{b}$ be vectors such that  $\|\mathbf{a}\| = 5,$$\|\mathbf{a} + \mathbf{b}\| = 7,$and$\|\mathbf{a} - \mathbf{b}\| = 9.$ Find $\|\mathbf{b}\|.$