If $f(x)=-6 x+9$, find $f(3)$.

Step 1 ：The function \(f(x)\) is a linear function. When we substitute \(x=3\) into the function, we get \(f(3)=-6*3+9\).

Step 2 ：After substituting \(x=3\) into the function, we get \(f(3)=-9\).

Step 3 ：Final Answer: \(\boxed{-9}\).