Question

Let $a^{-}$ $,b^{-}$ and $c^{-}$  be three unit vectors such that $a^{-}×\left(b^{-}×c^{-}\right)=\frac{\sqrt{3}}{2}\left(b^{-}+c^{-}\right)$ if $c^{-}$ is not parallel to is $c^{-}$ , then the angle between $a^{-}$ and $,b^{-}$ is: