How Do You Calculate the Angle Between Two Forces in Equilibrium?

[BioBabe91]

Homework Statement

Forces of 5N, 7N, and 8N are applied to an object, calculate the angle between the lines of action of the 5N and 7N forces.

Homework Equations

Sin, cos, and tan relationships

The Attempt at a Solution

I know that you have to use components, but I don't see how to do this without knowing at least one angle.

 

[rl.bhat]

According the cosine rule
a^2 = b^2 + c^2 - 2bc*cosθ. In the problem a, b and c is given. Find θ.