Question

focal length of lens may be determined by the given power of the lens

Answer 1

Power of the lens is the reciprocal of its focal length; i.e,
Power of a lens= 1/f where f is the focal length

The power of the lens is given the unit of Diopters (D).

Answer 2

Power of the lens is defined as the  convergence / divergence power of the lens..
    The amount of deviation o f  light rays = Tan Ф..

  P = 100/f  Dioptres    when f is expressed in cm.
     = 1/f  Dioptres    when f is in meters

   focal length =  1/P    in meters.     with P in Dioptres.
       f can be negative  if P is negative.  for a concave lens.
       f is positive for a convex lens.