We need the radii of surfaces of lens R1 and R2. I am giving the formulas and the procedure here. So follow these steps.

__1) Formulas:__Back Effective focal length to focus F' = B EFL = f ' (+ve)

Effect focal length in front of the front surface = EFL = f (-ve)

R1 = +ve R2 = -ve

Thickness of the lens = d

μ = refractive index of the lens wrt air (medium) around the lens on both sides

Frontal focal length (distance) from surface to Focus F = FFL = -ve

Back Focal length (distance) from back surface to Focus F' = BFL = + ve

__2. Given values:__Given R1 = R2, d = 5 cm, μ = 1.5 , u = - 20 cm

__3. Procedure:__ 1) Find f = f'

2) Then find v from the formula 1/f = 1/v - 1/u

3) From v now subtract (f' - BFL) to get the distance of image from the rear surface. This value of (f ' - BFL) or (f - FFL) may be close to d/2.