Answer
$\displaystyle \frac{5}{7}x^{3}-\frac{9}{20}x$
Work Step by Step
$ \begin{array}{lllll}
& \frac{3}{7}x^{3} & -\frac{1}{5}x & -\frac{1}{3} & \\
-[ & (-\frac{2}{7})x^{3} & +\frac{1}{4}x & -\frac{1}{3} & ]\\
\hline & & & & \\
= & (\frac{3}{7}+\frac{2}{7})x^{3} & +(-\frac{1}{5}-\frac{1}{4})x & +(-\frac{1}{3}+\frac{1}{3}) & \\
& & & & \\
= & \frac{5}{7}x^{3} & +\frac{-4-5}{20}x & +0 & \\
& & & & \\
= & \frac{5}{7}x^{3} & -\frac{9}{20}x & & \\
& & & &
\end{array}$