7-x=3
x=7-3
x=4
7-4=3
6-x=2
x=6-2
6-4=2
8-x=3
x=8-3
x=5
8-5=3
12-x=7
x=12-7
12-5=7
11-x=2
x=11-2
x=9
11-9=2
21-x=12
x=21-12
21-9=12
7-x=3
x=7-3
x=4
7-4=3
6-x=2
x=6-2
x=4
6-4=2
8-x=3
x=8-3
x=5
8-5=3
12-x=7
x=12-7
x=5
12-5=7
11-x=2
x=11-2
x=9
11-9=2
21-x=12
x=21-12
x=9
21-9=12