Hi

calculate the back vertex power of a lens made from glass n = 1.5. with an axial thickness of 9mm if the front surface has a power of +10.76D and the back surface has a power of -3.00D?

so far I have:

n= 1.5
t= 9mm
F1= +10.76D
F2= -3.00D
F= +7.76D

I used this formula: t/n which is 9/1.5 = 6mm

if someone could point out some guidance, that would be great. thanks