Given the following multiple regression equation:

hat$(y)=70-7x_1+16x_2$

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

a$)$ If $x_1$ increases by $1$ unit while holding $x_2$ constant, then $y$ by units on average.

b$)$ If $x_2$ increases by $1$ unit while holding $x_1$ constant, then $y$ by units on average.