At the movies, my friend bought a drink and popcorn. The total cost of my friend's purchase was $15. The drink cost $6. Which equation represents the total cost of my friend's purchase, along with the cost of the popcorn?

15 - 6 = p; p = 21

p + 15 = 6; p = 9

6 + 15 = p; p = 21

p = 15 - 6; p = 9