The area of only one base was added. The first term should have a factor of 2.
Since the photo booth charges a $500 fee for two hours at the party and an additional 50 dollars per hour, we have:
50x+500=C
Where x is the hours stayed and C is the total cost. Since Cindy doesn't want to spend more than 700 dollars, we have:

So she spent an additional 4 hours on the rental.
Usually you use addition to find a sum.