Answer:
1.
(a) The Domain is the set of inputs of the function.
Considering that the function takes a period of 3 weeks (21 days), the domain is [0, 21], once we can't evaluate what happens after the 21st day.
![\text{Domain is } [0, 21]](https://tex.z-dn.net/?f=%5Ctext%7BDomain%20is%20%7D%20%5B0%2C%2021%5D)
Otherwise, it could be ![[0, \infty)](https://tex.z-dn.net/?f=%5B0%2C%20%5Cinfty%29)
Note: We include 0 and 21.
Once the greatest balance was $400, it will not exceed $400, either it doesn't show negative values.
Note: We include 0 and 400.
(b)
Once the greatest balance was $400, when x=0, it seems that the y-value is half of $400, therefore, approximately $200. It also represents the initial value, the amount of money when she opened the account.
(c)
![f(x)=B(d)](https://tex.z-dn.net/?f=f%28x%29%3DB%28d%29)
![B(12)=0](https://tex.z-dn.net/?f=B%2812%29%3D0)
(d)
It is in segment 4.
The balance equal to zero means that the y-value of the graph is zero, therefore in the x-axis.