if cos θ=a, express 2θ in terms of a.
Use the identity $\displaystyle cos(2\theta) = 2cos^2(\theta) - 1$
(this is derived from the double angle formula for cos and the Pythagorean identity, both of which you'll need to know)
Knowing $\displaystyle cos^2 (\theta) = (cos(\theta))^2 = a^2$ you can solve it