The Question

Given a rectangular tetrahedron (i.e. a pyramid where each face is an equilateral triangle), find the cosine of the angle between two of the faces.

My Workings

To be honest I have not really got any worth showing for this one, I seem to be struggling with the concept of questions that ask for the angles between 2 planes. I'm just not sure how to approach them.

Apparently cos(60) is not the answer