Finding the depth of a parabolic reflector with 10cm diameter and 2cm focus

[dillonwerd]

This is my first post here so I hope I put it in the proper place.

Here's the basic info on the question.

parabolic reflector (like a tv dish) has diameter of 10cm, focus is 2 cm from the vertex. Find the depth.

Thanks in advance for an help. I've been stuck on this for a while! Getting frustrated.

 

[HallsofIvy]

First write down the equation for a parabola with y-axis a axis of symmetry, passing through (0,0), focus at (0, 2). Now, when x= 5, what is y?