Find intersection point between a ray and a face?

HOW are you given them?? If you are given their equations, then the equation of the plane should be something like: Ax+ By+ Cz= d while the ray would be given by parametric equations- x= at+ b, y= ct+ d, z= et+ f with t>= 0, say.
Put those equations for x, y, z, in terms of t, into the equation of the plane and solve the equation for t. If t>= 0, put it into the equations for the ray to find (x,y,z). If t<0 then the ray does not go through the plane- it goes the "away from" the plane.

(It is possible that the coefficient of t in the final equation will be 0. In that case the ray is parallel to the plane.)