Ok, if each ticket is $30, and you pay a one-time handling fee of $1.50, then wouldn't you agree the price depends solely on how many tickets you buy? With that in mind, the variable goes with the price of each ticket:
f(x)=30x+1.5
Because no matter what, you'll pay the $1.50 just once.
Now, say 6 tickets are bought:
f(6)=30(6)+1.5
To simplify:
f(6)=180+1.5
f(6)=181.5
So to buy 6 tickets, you would have to form over $181.50
Hope this helps!