Multiple (3 variable) regression model

Question

In order to determine the parameters b1, b2 and b3, this chapter simplifies the writing using the "deviation form" (red part).

I can't understand why the term b1 disappears.

Here is the chapter mentioned above.

Answer 1

Just above equation 2.10, the author defines $x_i$ and $y_i$ as $X_i- \bar X$ and $Y_i- \bar Y$, respectively. This is what they by talking about the "deviation form", i.e., using the deviations of the variables from the means rather than the raw variables themselves. When both the predictors and outcome are centered, the intercept is equal to zero, so $\beta_1$ can be removed from the model.