Let $a$ and $b$ be vectors such that $||a|| = 5$, $||a+b|| = 7$, and $||a-b|| = 9$. Find $||b||$.
What I've Tried: ( assuming * $=$ using a dot product) I did some rearranging by using the difference of the $||a+b||$ and $||a-b||$ terms, and found that $a * b = -8$. However, I'm unsure on how to proceed from here to get ||b||. Any help would be greatly appreciated!