$\displaystyle

\mbox{Given that} \ sec\theta = 2 , tan\theta = -\sqrt{3} \ and \ -\pi <\theta< \pi

$

Find the exact value of the angle $\displaystyle

\theta

$ in radians and justify your answer.

I'm stuck on this one why can't I just use the arctan function on my calculator? I know I can't that would be too simple. Can any one give me a clue ?