Answer:
x = 9
Explanation:
given y varies directly with x then the equation relating them is
y = kx ← k is the constant of variation
to find k , substitute y = 36 , x = 4 into the equation
36 = 4k ( divide both sides by 4 )
9 = k
y = 9x ← equation of variation
when y = 81 , then
81 = 9x ( divide both sides by 9 )
9 = x