15,518, because it is over 15,500 and below 16,499.999999...
Answer:
X = 1
Step-by-step explanation:

Answer is x+9=11
6+x=9
x=3
15=x+12
x=3
7+x=10
x=3
x+9=11
x=2
So x+9=11 has a different x value than the rest of the equations. So it doesn't belong with the other three.