\[- \frac{5}{6}a + \frac{7}{8}b + \frac{7}{12}a - \frac{5}{12}b =\]
\[= \left( - \frac{10}{12} + \frac{7}{12} \right)a + \left( \frac{21}{24} - \frac{10}{24} \right)b =\]
\[- \frac{3}{12}a + \frac{11}{24}b = - \frac{1}{4}a + \frac{11}{24}b\]