Here's a hint to get you started: cos(2x)=cos(x+x).
If cos(x) * cos (2x) = 0.707[i.e 1/sqrt(2)]. Find x. I know the answer is approximately 20.5deg. (Approximated from 20.47deg]. I don't know how to get to this answer. Please explain in a step by step detailed manner. Thanking You in advance.