Posted by help please on .
Telephoto lens. Use the formula from Exercise 67 to find the image distance i for an object that is 2,000,000 mm from a 250mm telephoto lens. (Section 6.7(1/f = 1/0 +1/i))

Algebra 
drwls,
250 mm is the focal length, f.
2*10^6 mm (2km) is the object distance, which you call "o". (It is usually written Do.)
Solve the equation provided to compute the image distance "i".
1/i = 1/250  1/(2*10^9)
"i" will be only slightly larger than 250 mm