Answer
$= \frac{3-8x}{6x^{3}}$
Work Step by Step
1) Multiply a fraction with the other denominator to obtain a common denominator between the two fractions. In this case, we multiply the first fraction by $3/3$ and the second fraction by $2x/2x$ to obtain a common denominator of $6x^{3}$
2) Subtract
$= \frac{1}{2x^{3}} - \frac{4}{3x^{2}}$
$= \frac{1(3)}{6x^{3}} - \frac{4(2x)}{6x^{3}}$
$= \frac{3-8x}{6x^{3}}$
