Subtract $ \dfrac{3}{x^4} $ from $ \dfrac{-10}{x^3} $ to get $ \dfrac{ \color{purple}{ -10x^4-3x^3 } }{ x^7 }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ x^4 }$ and the second by $\color{blue}{ x^3 }$.

$$ \begin{aligned} \frac{-10}{x^3} - \frac{3}{x^4} & = \frac{ \left( -10 \right) \cdot \color{blue}{ x^4 }}{ x^3 \cdot \color{blue}{ x^4 }} - \frac{ 3 \cdot \color{blue}{ x^3 }}{ x^4 \cdot \color{blue}{ x^3 }} = \\[1ex] &=\frac{ \color{purple}{ -10x^4 } }{ x^7 } - \frac{ \color{purple}{ 3x^3 } }{ x^7 }=\frac{ \color{purple}{ -10x^4-3x^3 } }{ x^7 } \end{aligned} $$