For this case we have to:
x: It is the variable that represents the number of compact discs.
The cost function must be found considering that each disk costs $7.13. So:
[tex]C (x) = 7.13x[/tex]
Thus, to find the total cost of "x" compact discs (in $) we substitute the number of discs in the cost function.
Answer:
[tex]C (x) = 7.13x[/tex]