Question

how can we calculate the power of the lens

Answer 1

First find the focal length(if it is not given). Generally it is given to you
Convert the focal length into meter. Let it be f meter

Power = 1/f D
D is the unit of power of lens. It is called diopter

Answer 2

Dn = u-1/r1 + u-1/r2 , D = i/f