et the width be x
"the length of the deck to be 5 ft longer than twice its width" --- >length = 2x+5
then
x(2x+5) = 250
2x^2 + 5x - 250 = 0
(x - 10)(2x + 25) = 0
x = 10 or x is a negative, which makes no sense
width = 10
length = 25
check:
is the length 5 more than twice the width ?? yes
is the area 250 ? yes