a. Express cos 2 2 sin 2 in the form R cos (2 + ), where

a. Express cos 2θ − 2 sin 2θ in the form R cos (2θ + α), where R > 0 and 0 < α <  π/2. Give the value of α to 3 decimal places. 

b. Hence, or otherwise, solve for 0 ≤ θ < π, cos 2θ − 2 sin 2θ = −1.5, rounding your answers to 2 decimal places.