2 identical glass Ug=3/2 equiconvex lenses of focal length f are kept in contact.the space between the two lenses is filled with water Uw=4/3.the focal length of the combination is

Hii

You can use the lens maker formula in this with proper sign convention in order to get the final answer.

You will get the radius of curvature from there you can find out the value of focal length