Answer
0.710 cm.
Work Step by Step
The image and object distances are related to the indices of refraction and to the radius of curvature.
$$\frac{n_a}{s}+\frac{n_b}{sā}=\frac{n_b-n_a}{R}$$
$$\frac{1.00}{40.0cm}+\frac{1.40}{2.60cm}=\frac{1.40-1.00}{R}$$
$$R=0.710cm$$