Angle Difference Identity Problem

[LevyOcampo]

Instructions:
Use the angle difference identity to find the exact value of each. ( i'll only be posting one problem)

cos -15°

I know that it will first equal to cos 15° because of the identity property (cos(-x) = cos(x)) but I dont know what to do after. I put the problem in a math solver online but the steps are hidden. I really want to understand how to reach the answer

 

[Dr.Peterson]

How can you express -15 as the difference of two "special angles" (30, 45, 60, 90)? (There are several ways.)

Do so, and apply the identity.