What is the approximate focal length for a 10X telescope with a 20 cm length?

[BuBbLeS01]

Test TOMORROW PLEASE HELP ME! Telescope!

Homework Statement

I need serious help! I have a test tomorrow and I am having trouble with microscope/telescope questions...

A person is designing a 10X telescope is limited to a length of 20 cm, what is the approximate focal length of the objective?

Homework Equations

The Attempt at a Solution

M = 10X
L = 20 cm
Fo = ?

The equation I found for a telescope is M = -Fo/Fe and the rest of the equations involve theta which I don't have. And I don't have Fe?

 

[BuBbLeS01]

I got it nevermind.