Your question is missing one part as "magnification"
i have completed the missing part below
Answer:
a. 
b. 
Explanation:
For this type of numerical we will use the following formulas
......... Eq1
where,

refractive index of the medium surrounding refracting surface/object
i.e.
=

refractive index of the refracting surface/object
i.e. 
distance of object from the vertex of the refracting surface
distance of image from the vertex of the refracting surface
radius of curvature of the refracting surface
........... Eq2
where,
magnification
Convention:

Given:
=






because refraction surface is concave
Required:
a. 

b. 
Solution:
a. putting values in eq1, we get



cm
b. 