to find x you first find the area of the school gym. (90*55) which would give you 4950. You take that from 5,500 and you end up with 550. now to get the right answer you divide by 55. and to prove this is correct ad 10 and 90. then multiply by 55. and you get the maximum length of 5500and since you already know the max you can use anything under it.
or 
or 