let
x----> the length side of the hypotenuse
y----> the length side of the unknown leg
we know that
x=2*y----> equation 1
applying the Pythagoras Theorem
x²=y²+3²-----> equation 2
substitute equation 1 in equation 2
[2*y]²=y²+3²----> 4*y²-y²=9-----> 3*y²=9----> y²=3
y=√3 ft
x=2*y----> x=2*√3 ft
the answer is
the lengths of the three sides are
hypotenuse=2√3 ft
one leg=√3 ft
other leg=3 ft
