Question 916049
The problem state "To rent a DVD, a customer must pay $3.99 plus $0.99 per day that the DVD is late." in the first sentence.  From this the dependent variable the # of days and 3.99 is the constant.  Therefore, the equation can look something like this: 

    f(d) = 3.99 + 0.99(d) 


Here's another example similar...

  The admission fee to a local carnival is $8. Each ride cost $1.50.  The total
cost depends on the number of rides ridden plus $8.  Let r = # rides ridden. 
What is the function?    the function f(r) = 1.50r + 8