А) (-2 5/6+3 5/24): 4 1/2; б)(1 2/3+2 5/9): (-21 3/4+7 1/2).

(-17/6 + 77/24):9/2 = (-68/24 + 77/24):9/2 = 9/24 * 2/9 = 1/12

(5/3 + 23/9):(-87/4 + 15/2) = (15/9 + 23/9):(-87/4 + 30/4) = 38/9 : (-57/9) = 38/9 * (-9/57) = - 38/57