Answer
$-\displaystyle \frac{3}{x}$
Work Step by Step
To subtract rational expressions with the same denominator,
subtract numerators and place the difference over the common denominator.
If possible, factor and simplify the result.
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Don't forget to place the second numerator in parentheses when subtracting.
$\displaystyle \frac{x^{3}-3}{2x^{4}}-\frac{7x^{3}-3}{2x^{4}}= \displaystyle \frac{x^{3}-3-(7x^{3}-3)}{2x^{4}}$
$= \displaystyle \frac{x^{3}-3-7x^{3}+3}{2x^{4}}$
$= \displaystyle \frac{-6x^{3}}{2x^{4}}$
$=\displaystyle \frac{2x^{3}(-3)}{2x^{3}(x)}$
... reduce (divide the numerator and denominator with common factors)
$=-\displaystyle \frac{3}{x}$