There are two thin symmetrical lenses: one is converging, with refractive index n1 = 1.70 and the

Question:

There are two thin symmetrical lenses: one is converging, with refractive index n1 = 1.70 and the other is diverging with refractive index n2 = 1.51. Both lenses have the same curvature radius of their surfaces equal to R = 10 cm. The lenses were put close together and submerged into water. What is the focal length of this system in water?