You know that 7/12 = 3/4 - 1/6 = 9/12 - 2/12.
You have memorized the exact values for the sine and cosine of the (1/4)(pi) and (1/6)(pi).
You have been given some sum and difference identities.
So restate the (7/12)(pi) in terms of (3/4)(pi) and (1/6)(pi), apply the appropriate identities, and then simplify the result, using what you know about the periodicity of the sine and cosine waves.
If you get stuck, please reply showing how far you have gotten in working through the steps. Thank you!