Angle Between Two Faces of a Triangular Pyramid A regular triangular pyramid has all of edges of length 36. What is the angle between any two faces of this pyramid?

I have no idea what to do for this problem. Can someone explain to me what an angle between two faces is?
The lengths of the sides of the regular tetrahedron are irrelevant.

Hint:  Find the vectors from the tetrahedron's center to the center of two faces.  Then calculate the angle between them.
This is asking for the angle defect between the two faces. To find the angle defect or the answer to your question, do 360 degrees minus the sum of the interior angles meeting at the vertex of where the two faces meet. The answer of that will be the answer to your question.