$\begin{align}  & f(x)={{x}^{2}}-2x\Rightarrow f(1-\sqrt{2})={{(1-\sqrt{2})}^{2}}-2(1-\sqrt{2})=1-2\sqrt{2}+2-2+2\sqrt{2}=1 \\  & f(-1)={{(-1)}^{2}}-2(-1)=1+2=3 \\  & \Rightarrow f(1-\sqrt{2})-f(-1)=1-3=-2 \\ \end{align}$