L=2W+3, A=LW, using L from the first in the second gives you:
A=(2W+3)W
A=2W^2+3W, and we are told A=90 so
2W^2+3W=90
2W^2+3W-90=0
2W^2-12W+15W-90=0
2W(W-6)+15(W-6)=0
(2W+15)(W-6)=0, since W>0 for all real possibilities,
W=6ft, and since L=2W+3
L=15ft
So the pool will be 6 ft wide by 15 ft long.
