$\begin{align}  & x-3=a\Rightarrow x=a+3 \\  & \Rightarrow f(a)={{(a+3)}^{2}}-4(a+3)+5 \\  & ={{a}^{2}}+6a+9-4a-12+5={{a}^{2}}+2a+2 \\  & \Rightarrow f(x)={{x}^{2}}+2x+2 \\  & \Rightarrow f(1-x)={{(1-x)}^{2}}+2(1-x)+2 \\  & =1+{{x}^{2}}-2x+2-2x+2={{x}^{2}}-4x+5 \\ \end{align}$