set up two seperate equations
first the volume equation
1728=length*width*height
we know the height is 6ft
1728=L*W*6
divide both sides by 6
288=L*W
then the second equation
L=4.5W
if one side is 4.5 times larger
then take that and plug it back into the first equation
288=(4.5W)*W
288=(4.5)w^2
divide both sides by 4.5
64=w^2
so w=8
then plug that back into the last equation
L=4.5(8)
L=36