The exercise gives us a function that describes the cost in dollars of buying x tickets to an ice hockey game, using a $25 discount coupon.
In the formula, C(x) represents cost, in dollars, and x represents number of tickets. Gordie buys four tickets, one for himself and one each for three friends. Calculating the cost of buying four tickets is the same as evaluating C(x) for x=4. To do so, we substitute 4 for x in the function.
With a $25 coupon, it costs $275 to buy 4 tickets to the orchestra.
In Part A, we were given the number of tickets and needed to calculate the cost. This time we are given the cost and want to calculate the number of tickets.
Calculating the number of tickets that can be bought with $800 means that we will need to substitute this amount for C(x) in the function, and solve for x.