Question

Application of Parabola:
A large reflecting telescope has a parabolic mirror that is 4 meters in diameter and 25 centimeters deep. It requires a cage for the observer with an eyepiece at the focus of the mirror. How far up the tube of the telescope must the cage be built? This design is called a prime focus telescope.

please help me by the use of Analytic geometry...

the answer must be 6inches away from the axis of the parabola.

thankS...