I need to prove that this implicit and multisteps scheme is consistent or not: $$y_{k+1}=\frac{1}{8}[9y_k-y_{k-2}+3hf(x_{k+1},y_{k+1})+6hf(x_k,y_k)-3hf(x_{k-1},y_{ k-1}) ]$$
but I'm a little lost because this is a multistep method and I have no light to be displayed in this case.
$\textbf{Edit:}$
The scheme is for compute a solution of a initial value problem:
$$y'(x)=f(x,y(x)) \ \ \ x\in [a,b]$$ $$y(a)=\alpha$$
And one method is consistent if the global truncation error tends to zero as $h$ (step size) tends to zero.
Thank you for your suggestions.