Part A:
For this case, the rate of change is given by the slope of the straight line.
We have then:

Substituting values we have:

Rewriting:

On the other hand, the initial value of the function occurs when x = 0.
We then have that for x = 0 the value of the function is 150.
Therefore we have:
Answer: m=-30, b = 150 The slope of the line represents the number of songs downloaded per week.
The cut-off point with the y-axis represents the initial number of songs that must be downloaded
Part B:
For this case the generic equation of the line is given by:

Where,
m: slope of the line
b: cutting point with the y axis.
Substituting the values of the part we have:
Answer: An equation in slope-intercept form to model the relationship between x and y is:
