\[\frac{12a - 4b}{a} \cdot \left( \frac{a^{\backslash b}}{3a - b} + \frac{a^{\backslash 3a - b}}{b} \right) =\]
\[= \frac{12a - 4b}{a} \cdot \frac{ab + 3a^{2} - ab}{b(3a - b)} =\]
\[= \frac{4 \cdot (3a - b) \cdot 3a²}{ab(3a - b)} = \frac{12a}{b}\]