so remember that the pythagorean theorem is a^2+b^2=c^2
c=18
legnth is 3 more than width
l=3+w
a=3+b
subsitute
(3+b)^2+b^2=18^2
expand
9+6b+b^2+b^2=324
subbtract 9 from both sides
6b+2b^2=313
subtract 313 from both sides
2b^2+6b-313=0
factor
use quadratic formula and get
b=11.1
subsitute
a=3+b
a=3+11.1
a=14.1
answer is 14.1 ft by 11.1 ft
