(12 5\13+у)-9 9\13=7 7\13

(12 5/13 + у) - 9 9/13 = 7 7/13;

(161/13 + у) - 126/13 = 98/13;

161/13 + у = 98/13 + 126/13;

161/13 + у = 224/13;

у = 224/13 - 161/13;

у = 63/13;

(12 5/13 + 63/13) - 9 9/13 = 7 7/13;

(161/13 + 63/13) - 126/13 = 98/13;

224/13 - 126/13 = 98/13;

98/13 = 98/13.