Subtract $ \dfrac{3}{x+4} $ from $ \dfrac{2}{x^2+3x-4} $ to get $ \dfrac{ \color{purple}{ -3x+5 } }{ x^2+3x-4 }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the second fraction by $\color{blue}{x-1}$.

$$ \begin{aligned} \frac{2}{x^2+3x-4} - \frac{3}{x+4} & = \frac{ 2 }{ x^2+3x-4 } - \frac{ 3 \cdot \color{blue}{ \left( x-1 \right) }}{ \left( x+4 \right) \cdot \color{blue}{ \left( x-1 \right) }} = \\[1ex] &=\frac{ \color{purple}{ 2 } }{ x^2+3x-4 } - \frac{ \color{purple}{ 3x-3 } }{ x^2+3x-4 }=\frac{ \color{purple}{ 2 - \left( 3x-3 \right) } }{ x^2+3x-4 } = \\[1ex] &=\frac{ \color{purple}{ -3x+5 } }{ x^2+3x-4 } \end{aligned} $$