Answer
$F=f\times \frac{A}{a}$
Work Step by Step
According to Pascal's principle, the pressure at small piston is equal to the pressure at big piston. So:
$P_a=P_A$
Hence,
$\frac{f}{a}=\frac{F}{A}$
$F=f\times \frac{A}{a}$
This is the magnitude of the force that the larger piston sustains without moving.
