Answer
(a) $18mm$
(b) $14$
Work Step by Step
(a) We can find the focal length of the eyepiece as
$f_{eyepiece}=L-f_{objective}$
We plug in the known values to obtain:
$f_{eyepiece}=275-257=18mm$
(b) The magnification can be determined as
$M_{total}=\frac{f_{objective}}{f_{eyepiece}}$
We plug in the known values to obtain:
$M_{total}=\frac{257}{18}=14$