Need help solving this problem

Can you think of what angle it is whose sine is 1/2? It's not 45 degrees (pi/4 radians); that angle has a sine of sqrt(2)/2. But it's another one of those common angles whose sine and cosine you memorize when you first learn trigonometry.

Once you figure out which angle has a sine of 1/2, just set that equal to 2theta.