What do you mean by combined focal length?

Explanation

The reciprocal of combined focal length is equal to the sum of the reciprocals of focal lengths of number of all lenses placed together.
`therefore 1/f=(1)/(f_2)+1/(f_2)` where f-combined focal length
`f_1` - focal length of `1^(st)` lens,
`f_2` focal length of `2^(nd)` lens.