Answer:
The 6 pack
Step-by-step explanation:
I say this mainly because if you buy two 6 packs of soda that will add up to 4 dollars and 8 cents which is cheaper than the 12 pack.
For this you simply plug in to substitute 3 wherever you see an x!
f(3) = 6(3)+7
f(3) = 18 + 7
f(3) = 25