4) \(\frac{a}{5}-\frac{9-a}{5}=\)

\(\frac{a}{5}-\frac{9-a}{5}=\frac{a-(9-a)}{5}=\frac{a-9+a}{5}=\frac{2a-9}{5}\).

Ответ: \(\frac{2a-9}{5}\)