(12 5/13-Y)-9 9/13=7 7/13

12 5/13-y-9 9/13=7 7/13
-y=7 7/13-12 5/13+9 9/13
-y=5 1/13
y= - 5 1/13
12 5/13+5 1/13-9 9/13=17 6/13-9 9/13=7 7/13