The answer is 4.5, since 1 & 8 are in the middle you add those two numbers and divide by two so 1+8=9 and 9/2=4.5
Your answer would be 9 feet
81=area
9=base
_=height
and area is base x hight so you divide the base by the area so an equation would be 81=9x then you would flip it around so it would be 9÷81=x
x=9
Hope this helps!!