I can't seem to understand this question at all. It does not make sense to me.
The question is
Given $\left|\vec a\right| = 3, \left|\vec b\right| = 5$ and $\left|\vec a+\vec b\right| = 7$. Determine $\left|\vec a-\vec b\right|$.
I have tried finding $\left|\vec a+\vec b\right|$ using cosine rule such that $\left|\vec a+\vec b\right| = 7 = 3^2 + 5^2 - 2\cosθ$
Which failed as I clearly am unable to picture this question correctly in my head. If someone could explain this question (or maybe help me sketch it) that's be very helpful, thanks in advance.