18) -\frac{5}{6}a + \frac{7}{8}6 + \frac{7}{12}a -\frac{5}{12}6 =

18) $$\frac{-5}{6}a + \frac{7}{8} \cdot 6 + \frac{7}{12}a - \frac{5}{12} \cdot 6 = \frac{-5}{6}a + \frac{7}{12}a + \frac{7 \cdot 6}{8} - \frac{5 \cdot 6}{12} = \frac{-10a + 7a}{12} + \frac{42}{8} - \frac{30}{12} = \frac{-3a}{12} + \frac{21}{4} - \frac{5}{2} = \frac{-a}{4} + \frac{21}{4} - \frac{10}{4} = \frac{-a}{4} + \frac{11}{4}$$

Ответ: $$\frac{-a}{4} + \frac{11}{4}$$