6) $$ \frac{5}{c+3} - \frac{5c-2}{c^2+3c} = \frac{5}{c+3} - \frac{5c-2}{c(c+3)} = \frac{5c - (5c-2)}{c(c+3)} = \frac{5c - 5c + 2}{c(c+3)} = \frac{2}{c(c+3)} $$.
Ответ: $$\frac{2}{c(c+3)}$$