$$-\frac{5}{15}-\frac{4}{15}$$
Since $-\frac{5}{15}$ and $\frac{4}{15}$ have the same denominator, subtract them by subtracting their numerators.$$\frac{-5-4}{15}$$
Subtract $4$ from $-5$ to get $-9$.$$\frac{-9}{15}$$
Reduce the fraction $\frac{-9}{15}$ to lowest terms by extracting and canceling out $3$.$$-\frac{3}{5}$$