You can
put this solution on YOUR website!not sure what you are doing here, but it is way off. Too much. Too much :-)
Your H, L and W definitions are spot on, but it says the base area = 84. The base area = LW, so (x-3)(x+5)=84. Multiplying this out gives

(x+11)(x-9) = 0
so x+11 = 0 OR x-9=0
so x=-11 or x=9
So, out solution is the x=9, since we are after a height.
jon.