Answer:
[tex]\large\boxed{B.\ (f-g)(x)=-x-15}[/tex]
Step-by-step explanation:
[tex](f-g)(x)=f(x)-g(x)\\\\f(x)=2x-6,\ g(x)=3x+9\\\\\text{Substitute:}\\\\(f-g)(x)=(2x-6)-(3x+9)\\\\=2x-6-3x-9\qquad\text{combine like terms}\\\\=(2x-3x)+(-6-9)\\\\=-x-15[/tex]