Set up a proportion.
[tex]\frac{x}{14 + x} = \frac{9}{9 + 12}[/tex]
Now cross multiply.
[tex]x(9 + 12) = 9(14 + x)[/tex]
---> [tex]21x = 9x + 126[/tex]
Isolate variable x ----> [tex]21x - 9x = 126[/tex]
----->[tex]12x = 126[/tex]
Once again isolate variable x:
[tex]\frac{12x}{12} = \frac{126}{12}[/tex]
x = 21/2 or 10.5.
