If F is 400 N and S=2m^2, find the pressure(P) ?
S cannot be displacement because it has dimensions of (Length)^2 whereas displacement has dimensions of (Length). This means that S is an area of some kind although it is more usual to use A to denote an area
$\displaystyle P = \frac{F}{A}cos \alpha$ where $\displaystyle \alpha$ is the angle between the force and the vertical and will be measured in $\displaystyle Pa$ or $\displaystyle Nm^{-2}$
In your example it's probably a perpendicular force so $\displaystyle cos \alpha = 1$ and it reduces to $\displaystyle P=\frac{F}{A}$