\[\frac{a^{2} - 5a}{3a^{2} - 48} - \frac{3a - 10}{3a^{2} - 48} + \frac{6}{3a^{2} - 48} =\]
\[= \frac{a^{2} - 5a - 3a + 10 + 6}{3a^{2} - 48} =\]
\[= \frac{a^{2} - 8a + 16}{3\left( a^{2} - 16 \right)} =\]
\[= \frac{(a - 4)^{2}}{3(a - 4)(a + 4)} = \frac{a - 4}{3a + 12}\]