Question

Please help! If the telescope is 9 m deep and 12 m wide, how far is the focus from the vertex?

Answer 1

so, you have a parabola function

Answer 2

bearing in mind that
$$
\text{parabola form, with focus "p"}\\
(x-h)^2 = 4p(y-k)
$$

Answer 3

this one is moving "upwards", thus we use the "x" component squared form

Answer 4

so, your picture looks like above