Given the following multiple regression equation:

hat$(y)=29+x_1+8x_2-2x_3$

Select the direction of change in $y$ and enter a positive integer in the answer box.

a$)$ If $x_1$ increases by $4$ units while holding $x_2$ and $x_3$ constant, then $y$ by units on average.

b$)$ If $x_2$ increases by $5$ units while holding $x_1$ and $x_3$ constant, then $y$ by units on average.

c$)$ If $x_3$ increases by $6$ units while holding $x_1$ and $x_2$ constant, then $y$ by units on average.