Answer
a) right
b) 2.5
Work Step by Step
(a) We know that $ M_{total}=\frac{f_{obj}}{f_e}$. This equation shows that the lens with a greater focal length or lower refractive power is used as the objective lens. In the given data, the right lens has the lower refractive power and larger focal length and hence the right lens is used as the objective lens.
(b) We can find the required angular magnification as follows:
$ M_{total}=\frac{P_L}{P_R}$
We plug in the known values to obtain:
$ M_{total}=\frac{5.0diopters}{2.0diopters}$
$ M_{total}=2.5$
