Answer
(a) $90V$
(b) $18V$
Work Step by Step
(a) We know that
$\Delta V=E\Delta S$
We plug in the known values to obtain:
$\Delta V=(1.2\times 10^5)(0.75\times 10^{-3})$
$\Delta V=90V$
(b) We know that
$\Delta V=E\Delta S$
We plug in the known values to obtain:
$\Delta V=(2.4\times 10^4)(0.75\times 10^{-3})$
$\Delta V=18V$