11
x-x/3=-1/3
2x/3=-1/3
x=-1/3:2/3
x=-1/3*3/2
x=-1/2
12
x-x/12=-11/12
11x/12=-11/12
x=-11/12:11/12
x=-1
13
x-x/9=10/3
8x/9=10/3
x=10/3:8/9
x=10/3*9/8
x=15/4
x=3,75
14
x-x/7=-39/14
6x/7=-39/14
x=-39/14:6/7
x=-39/14*7/6
x=-13/4
x=-3,25
15
x-x/7=-3/14
6x/7=-3/14
x=-3/14:6/7
x=-3/14*7/6
x=-1/4