a = - 2/21
b = - 5/7
a/b = - 2/21 : - 5/7 = - 2/21 * - 7/5 = 14 / 105
b /a = - 5/7 : - 2 /21 = - 5 /7 * - 21/2 = 105/ 14
ab = - 2 /21 * - 5 /7 = 10 / 147
a + b = - 2/21 - 5 /7 = - 2 /21 - 15/21 = - 17/21
a - b = - 2 /21 + 5/7 = - 2/21 + 15/21 = 13 /21