A box without a lid is constructed from a 24 inch by 24 inch piece of cardboard by cutting $x$ in. squares from each corner and folding up the sides.
a) Determine the volume of the box as a function of the variable $x$.

$V(x) =$
b)Use a graphing calculator to approximate the values of $x$ that produce a volume of 972.
Note: There are 3 values of $x$ that produce the given value but only two of them are acceptable in the context of the problem. List the two answers, to at least one decimal place, separated by commas.

$x =$