Answer: We should add the constant 81 to the expression to have a perfect square trinomial.
The perfect square trinomial that would be formed would result from (x + 9)^2.
We can use foil to prove it.
(x + 9)(x + 9)
x^2 + 9x + 9x + 81
x^2 + 18x + 81
81 is the value that must go with 18x in the middle to form the perfect square trinomial.