[m](-\frac{5}{6}a+\frac{7}{12}a)+(\frac{7}{8}b-\frac{5}{12}b)=(-\frac{5\cdot 2}{6\cdot 2}a+\frac{7}{12}a)+(\frac{7\cdot 3}{8\cdot 3}b-\frac{5\cdot 2}{12\cdot 2}b)=(-\frac{10}{12}a+\frac{7}{12}a)+(\frac{21}{24}b-\frac{10}{24}b)=-\frac{3}{12}a+\frac{11}{24}b[/m]
