Solve this following

If a unit vector $\overrightarrow{\mathrm{a}}$ makes angles $\pi / 3$ with $\hat{\mathrm{i}}, \pi / 4$ with $\hat{\mathrm{j}}$ and $\theta \in(0, \pi)$ with $\hat{\mathrm{k}}$, then a value of $\theta$ is :-