You can
put this solution on YOUR website!let the length be x and width be y
then
x=2y-10.......[1]
2(x+y)=400....[2]
substituting [1] in [2] we get
2(2y-10+y)=400
(2y-10+y)=200
3y-10=200
3y=210
y=70
x=2y-10=140-10=130
Dimensions are: 70 yards,130 yards
Hope this helps,
Prabhat