Using the given formula, the length is equal to $\displaystyle\frac{{{28}{{x}}^{{3}}}}{{{4}{x}}}\usepackage{cmbright}\Huge\$ . Simplify as follows:

$\displaystyle\frac{{{28}{{x}}^{{3}}}}{{{4}{x}}}={\left(\frac{{28}}{{4}}\right)}\cdot\frac{{{{x}}^{{3}}}}{{{x}}}={7}\cdot{{x}}^{{{3}-{1}}}={7}{{x}}^{{2}}\usepackage{cmbright}\Huge\$