Subtract $ \dfrac{a}{5} $ from $ \dfrac{a}{25} $ to get $ \dfrac{ \color{purple}{ -4a } }{ 25 }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the second fraction by $\color{blue}{5}$.

$$ \begin{aligned} \frac{a}{25} - \frac{a}{5} & = \frac{ a }{ 25 } - \frac{ a \cdot \color{blue}{ 5 }}{ 5 \cdot \color{blue}{ 5 }} = \\[1ex] &=\frac{ \color{purple}{ a } }{ 25 } - \frac{ \color{purple}{ 5a } }{ 25 }=\frac{ \color{purple}{ -4a } }{ 25 } \end{aligned} $$

Divide $ \dfrac{-4a}{25} $ by $ a $ to get $ \dfrac{ -4a }{ 25a } $.

Step 1: To divide rational expressions, multiply the first fraction by the reciprocal of the second fraction.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{ \frac{-4a}{25} }{a} & \xlongequal{\text{Step 1}} \frac{-4a}{25} \cdot \frac{\color{blue}{1}}{\color{blue}{a}} \xlongequal{\text{Step 2}} \frac{ \left( -4a \right) \cdot 1 }{ 25 \cdot a } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ -4a }{ 25a } \end{aligned} $$