The perimeter, $P$, of a square is a function of the length of its side, $s$.
a) Write an equation using function notation to describe this relationship between $P$ and $s$.
b) If the length of a side is given in feet, find $P(2)$ and explain what this means in the context of the problem.
c) If the length of a side is given in centimeters, find $P(11)$ and explain what this means in the context of the problem.
d) What is the length of each side of a square that has a perimeter of 18 inches?