Does someone know how to do math with that equation

f = 5 cm

Explanation:

using the thin lens equation, given as follows:

where,

f = focal length = ?

do = the distance of object from lens = 20 cm

di = the distance of image from lens = 6.6667 cm

Therefore,

f = 5 cm

answer; how far out of place which the object finished relative to the starting location;