Iam stuck on this algebra question:

a homeowner wants to increase the size of his deck that now measures 15 feet by 18 feet. his homeowner's association declares that no deck is to be more than 928 square feet. if the length and the width of the deck are to be increased by the same amount, find, to the nearest tenth, the maximum number of feet by which the length of the deck may be legally increased.

a) 14 feet
b)21 feet
c)28 feet
d)32 feet