Answer:
I believe the answer is 6
Step-by-step explanation:
f(x) = 3x - 15 -- plug 7 into x (that's how many candy bars he's buying)
f(x) = 3(7) - 15 -- solve the equation (just know that 3(7) is the total, and 15 is the amount of money on his gift card.)
f(x) = 21 - 15
f(x) = 6 -- your answer
I hope this helps :)