Orthogonality & Least Squares · Medium problem

A $4$-sample audio signal $\mathbf s=(2,5,3,8)$ is filtered by projecting it onto the "clean" subspace spanned by the constant pattern $(1,1,1,1)$ and the linear-trend pattern $(-3,-1,1,3)$ (already orthogonal to each other), discarding everything else as noise. Find the magnitude of the discarded (residual) part.