Final answer:
The y-intercept of Sarah's package shipping service charge is $4, which is the fixed service charge she applies before shipping any items.
Step-by-step explanation:
The question is asking us to determine the y-intercept of a linear function based on the given information about Sarah's package shipping charges.
The y-intercept in a linear equation y = mx + b is represented by 'b', and it is the value of 'y' when the independent variable 'x' is zero. In the context of this scenario, this corresponds to the initial service charge that Sarah applies before any items are shipped.
According to the details provided, Sarah charges a fixed rate of $4 for her services, regardless of the number of items. This means that, if no items are shipped (x = 0), the total cost would be $4. Therefore, the y-intercept for Sarah's shipping service is $4.