$$y(n)=0.9955 x(n-2)-1.8936 x(n-1)+0.9955 x(n)-0.9911 y(n-2)+1.8936 y(n-1)$$ $$Y (z)=0.9955X (z-2)Z^{-2} - 1.8936 X (z-1)Z^{-1}+0.9955 X(z)-0.9911 Y(z)Z^{-2}+1.8936 Y(z)Z^{-1}$$ $$H (z)=\frac{Y (z)}{X (z)}=\frac{0.9955-1.8936Z^{-1}+0.9955Z^{-2}}{1-1.8936Z^{-1}+0.9911Z^{-2}}$$