Ok, if I know that $\displaystyle 3 \vec c = \vec a + 2 \vec b$ and $\displaystyle \vec d = - \vec a + 2 \vec b$ and that $\displaystyle | \vec a | = 2 | \vec b |$. (o is origin)

How do I prove that $\displaystyle |<cod| = \frac{ \pi }{2}$ ?

Any hints would be extremely helpful here. Thanks in advance.