Answer:
Yes.
Step-by-step explanation:
[tex]f(g(x))[/tex]
[tex]f(\frac{x-12}{9})[/tex]
( I will have to replace all [tex]x[/tex]'s in [tex]9x+12[/tex] with [tex](\frac{x-12}{9}[/tex])
[tex]9(\frac{x-12}{9})+12[/tex]
[tex](x-12)+12[/tex]
[tex]x-12+12[/tex]
[tex]x+(-12+12)[/tex]
[tex]x+0[/tex]
[tex]x[/tex]
So the answer is yes.