b - is a part of segment = 9.
We can see that
9 =b + x
b+x+b =12 -----> 2b + x =12
b +x=9 /*(-2) ------> -2b - 2x = -18
2b+x=12
-2b - 2x = -18
2b + x = 12
-x = -6
x = 6
