We must get the equation.

Let y = the total cost and x = the number of cds.

So when x = 5, y = $12 and
   when x = 9, y = $20

So the problem becomes:

Find the equation of the line through the points (5,12) and (9,20)

Slope formula:

m = 

where (x1,y1) = (5,12)

and where (x2,y2) = (9,20)

m = 

m = 

m = 2

We use the point-slope formula:

y - y1 = m(x - x1)

where (x1,y1) = (5,12) and m = 2

y - 12 = 2(x - 5)

y - 12 = 2x - 10

     y = 2x + 2

To answer:

Substitute x = 50

     y = 2x + 2

     y = 2(50) + 2    

     y = 100 + 2

     y = 102

Answer $102.

Edwin