If they started of paying 15 then that's your constant. So what you do is subtract 15 from 45 and see what's left which will be 30. Since each bag of popcorn is $2 you divide the $30 dollars you have left over by 2 and you'll get 15 for the amount of popcorn bags they'll have to buy.
The pattern is n-3, so we will start from the beginning, from 1. 7, 4, 1, -2, -5, -8, -11, -14, -17, -20, -23, -26, -29, -32, -35, -38, -41, -44. -44 is the 18th term in the sequence.
