Power P_{1} = -3.5D; P_{2} = +1D

Total power = P_{1} + P_{2} = -3.5D + 1D

⇒ Total power (P) = -2.5D.

**Now; **

P (in diopter) = \(\frac{1}{f(in\,meters)}\)

⇒ -2.5 = \(\frac{1}{f(in\,meters)}\)

⇒ f (in meters) = \(\frac{-1}{2.5}\,=\,\frac{-1}{\frac{5}{2}}\)

= f (in meters) = - \(\frac25\)= -0.4m

Now 1m = 100cm

⇒ -0.4m = -40cm

**Hence the focal length of lens is -40cm.**