The lens, –1.75DS / +1.75DC × 180 is made in glass, n = 1.56, in toric form
with a –4.00D base curve and edged 40mm round. The thick-edge thickness
is 3.14mm. What is the thin-edge substance of the lens?

I bealive i have to find the sag of +1.75; -1.75 and -4.00 (of course without the sign, but then i don't know the formula to find the thin edge subsatnce.

please i need help!!!

Thanks for reading.