(a) To write a linear function that describes the cost of studio membership, we can use the point-slope form of the equation of a line:
c - c1 = m - m1
where c1 and m1 are the cost and number of months for one of the data points, and m is the number of months. We can use either of the data points to find the equation, but let's use the data for your friend:
c - 645 = (m - 15)(852 - 645)/(24 - 15)
Simplifying, we get:
c = 90m - 645
(b) The cost per month is the slope of the line, which is 90. The joining fee is the y-intercept of the line, which is -645 (since the cost is negative when m = 0, which represents the joining fee).
(c) To determine the maximum number of months for $1500, we can set the cost equal to $1500 and solve for m:
1500 = 90m - 645
2145 = 90m
m = 23.83
Since we can't stay for a fraction of a month, the maximum number of months we can stay for $1500 is 23 months.