Answer
The volume of the package in terms of the length of a side of its square front $x$ is $V=300{{x}^{2}}-4{{x}^{3}}\text{ cubic inches}$.
Work Step by Step
Consider the length plus girth of the provided package: $\text{300 inches}$
Now, the measurement of the length plus girth is the sum of the distance around that package and the longest side of girth
$4x+y=300$
Or
$y=300-4x$
The volume of provided package, $V=l\cdot w\cdot h$
Substitute $y$ for $l$ , and $x$ for $w$ and $h$ in the above equation
$\begin{align}
& V=y\cdot x\cdot x \\
& =y\cdot {{x}^{2}}
\end{align}$
Substitute $300-4x$ for $y$ in the above equation
$V=\left( 300-4x \right)\cdot {{x}^{2}}$
Or
$V=300{{x}^{2}}-4{{x}^{3}}$
The required volume of the package in terms of the length of a side of its square front $x$ is, $V=300{{x}^{2}}-4{{x}^{3}}\text{ cubic inches}$.
