You wish to form an image of the moon, diameter 3500 km, a distance of 380 000 km away, with a reflecting telescope. If the image needs to be 4.5 cm in diameter, find the required focal length of the mirror in the telescope.

Formulas are
$\displaystyle \frac{1}{f}=\frac{1}{di}+\frac{1}{do}$
$\displaystyle \frac{hi}{ho}=\frac{di}{do}$
F stands for focal point, di is distance to the image, do is distance to the object, hi is height of the image and ho is height of the object.
$\displaystyle \frac{4.5*10^5}{3500}=\frac{di}{380000}$
$\displaystyle f=4.88*10^3$
$\displaystyle \frac{1}{f}=\frac{1}{380000}+\frac{1}{4.88*10^3}$
$\displaystyle f= 4.88*10^3 km $
is this correct???