1. I assume that the a in cos(a) denotes the angle opposite the side a. To avoid some confusion I'll use to denote this angle. Consequently denote the angles opposite the sides b or c.

2. Use a triangle with the side lengthes

a = 7, b = 8, c = 9

3. Use the Cosine rule to determine the cosine values of the corresponding angles.

I've got: