See buddy....what I feel is........

this can be considered as a case of lenses in contact with each other.......

here we have...2 similar plano-convex lenses kept in opposite directions to each other........so their powers will simply cancel each others effect and we only need to consider one concave lens made out of liquid...kept in medium of glass....whose focal length will give us the focal length of the combination.....

that was the concept I used.....If u find any mistake then pls let me know>>>>>