Parallelogram law: $\|u+v\|^2+\|u-v\|^2=2\|u\|^2+2\|v\|^2$

Let $u$ and $v$ be vectors in an inner product space such that $\|u+v\|=8$, $\|u-v\|=6$ and $\|u\|=7$. Then $\|v\|=$