Answer
$f = -40~cm$
Work Step by Step
We can find the focal length:
$\frac{1}{f} = (n-1)(\frac{1}{R_1}-\frac{1}{R_2})$
$\frac{1}{f} = (1.50-1)(\frac{1}{-40~cm}-\frac{1}{40~cm})$
$\frac{1}{f} = (0.50)(\frac{-2}{40~cm})$
$\frac{1}{f} = -\frac{1}{40~cm}$
$f = -40~cm$
