Compare the argument showing that $\hat{\beta}_R = \underset{\beta}{\text{argmin}} \ ||\beta - \delta y + s||^2 - \mu - X\beta||^2$ with equation (4.10) with the derivation of $\hat{\beta}_1 = \underset{\beta_1}{\text{argmin}} \ ||Y_{1.2} - X_{1.2}\beta_1||^2$ starting on page 61.