It can be solved by simple Lens Maker formula
1/f = (µ-1)(1/R1 - 1/R2)
As the type of lens is not given, so we can find the focal length for both convex and concave lens.
For Convex lens: R1 = 0.7 m and R2 = − 0.7 m
For Concave Lens: R1 = -0.7 m and R2 = 0.7 m
Refractive index µ will be same for the lens i.e µ = 1.8
Now we can find the focal length for both convex and concave lens by putting the values of R1 and R2 in above lens formula.