Step 1 :Given the function \(f(x)=x-6\).
Step 2 :Substitute \(x=8\) into the function to get \(f(8)=8-6\).
Step 3 :Simplify to get \(f(8)=2\).
Step 4 :Substitute \(x=5\) into the function to get \(f(5)=5-6\).
Step 5 :Simplify to get \(f(5)=-1\).
Step 6 :So, the final answers are \(f(8)=\boxed{2}\) and \(f(5)=\boxed{-1}\).