I would say you could purchase 3 balloons.

If each balloon costs $2 and you need to pay a $3 delivery fee, then you would set up an equation:

p = $2x + $3 where p is the total price and x is the number of balloons.

Now we would substitute different numbers in for x:

p = $2(1) + $3

p = $2 + $3

p = $5

p = $2(2) + $3

p = $4 + $3

p = $7

p = $2(3) + $3

p = $6 + $3

p = $9

Now, knowing that we have $9 to spend, the max amount of balloons we can buy is 3. :)