A spotlight has a parabolic cross section that is 4 ft wide at the opening and 1.5 ft deep at the vertex, how far is the vertex from the focus?

I'm quite confused as my textbook gives no illustration, im assuming that the light is 4ft wide and 1.5ft deep from the opening/plexiglass.

Saying the light is on a cartesian coordinate system with vertex (0,0) gives me no clue as to how this question is solved.

Answer is 2/3, how did you get such an answer??