Answer:

And using the minutes from June we can find the value for b the intercept


And the function would be:

Step-by-step explanation:
For this case we define the following notation first:
C represent the cost for a given month
x represent the number of minutes of calling time she used
And we want to find a function of C in terms of x
We have the following info:
June
x =100 , C = 54
July
x =660 , C = 138
For this case we can assume that we can use a linear function since we have just two points given, and the general formula would be:

Where a is the slope and can be estimated as:

And using the minutes from June we can find the value for b the intercept


And the function would be:
