A circular 6 sliced pizza is placed in a square be $x$ that has dimensions 3 inches larger than the diameter of the pizza. If the box covers a surface area of $400\; in^2$, what is the surface area of one slice of pizza ?

Question

$\begin{array}{1 1} (a)\; 6 \; in^2 \\ (b)\; 10 \;in^2 \\ (c)\; 12\;in^2 \\ (d)\; 14 \; in^2 \end{array}$