I have no idea what this question is asking or how to do it please help?

Find the angle between the diagonal of a cube of side length 12 and the diagonal of one of its faces, so that the two diagonals have a common vertex. The angle should be measured in radians. (Hint: we may ume that the cube is in the first octant, the origin is one of its vertices, and both diagonals start at the origin.)