The moon is 3.5106m3.5106m in diameter and 3.8108m3.8108m from the earth's surface. The 1.3-mm-focal-length converging mirror of a telescope focuses an image of the moon onto a detector.

What is the diameter of the moon's image?

I used the equation 1/f =i/o+1/o

1/i=1/1.3

-1.3/(3.8*10^8)

= -3.4210*10^-9

I'm not sure what to do after this step