Tickets cost $0.75t. Kendall bought 18 tickets, so you can substitute 18 for the variable t because it stands for the amount of tickets bought.
$0.75(18) = $13.50
Kendall spent a total of $33.50, so that means she spent $13.50 on tickets and the rest she spent on admission, so 33.50 - 13.50 = 20; she spent $20.00 on admission.
(a) The variable are t, amount of tickets bought, and a, admission price.
(b) The linear equation for anyone who only pays for festival admission and rides would look like: 0.75t + a.
(c) I made the linear equation 0.75t + a because first you would have to pay the admission fee (a), and then you would pay for EACH ticket (t). So t would be the changing variable depending on how many tickets a person would choose to buy.