A)

[tex]A = - \frac{7}{12} + \frac{5}{4} \\ = \frac{ - 7}{12} + \frac{5 \times 3}{4 \times 3} \\ = \frac{ - 7}{12} + \frac{15}{12} \\ = \frac{ - 7 + 15}{12} \\ = \frac{8}{12} \\ = \frac{2 \times 2 \times 2}{2 \times 2 \times 3} \\ = \frac{2}{3} [/tex]

B)

[tex]B = - \frac{3}{7} + \frac{6}{7} \times \frac{5}{2} \\ = \frac{ - 3}{7} + \frac{6 \times 5}{7 \times 2} \\ = \frac{ - 3}{7} + \frac{3 \times 2 \times 5}{7 \times 2} \\ = \frac{ - 3}{7} + \frac{3 \times 5}{7} \\ = \frac{ - 3 + 15}{7} \\ = \frac{12}{7} [/tex]

C)

[tex]C = \frac{3}{7} \times \frac{ - 14}{9} \\ = \frac{3 \times ( - 14)}{7 \times 9} \\ = \frac{3 \times ( - 1) \times 7 \times 2}{7 \times 3 \times 3} \\ = \frac{( - 1) \times 2}{3} \\ = - \frac{ 2}{3} [/tex]

D)

[tex]D = ( \frac{1}{3} - 2) \div ( \frac{1}{3} + 2) \\ = ( \frac{1}{3} - \frac{2 \times 3}{3} ) \div ( \frac{1}{3} + \frac{2 \times 3}{3} ) \\ = ( \frac{1 - 6}{3} ) \div ( \frac{1 + 6}{3} ) \\ = \frac{ - 5}{3} \div \frac{7}{3} \\ = \frac{ \frac{ - 5}{3} }{ \frac{7}{3} } \\ = \frac{ - 5}{3} \times \frac{3}{7} \\ = \frac{ - 5}{7} [/tex]