Let's look at this from a slightly different perspective
Let the interger in the exact middle of the two integers (their average) = x
Then the lesser number = (x - 9)
And the greater = (x + 9)
Note that there is a difference of 18 units between the integers
Note that their product is ( x - 9) ( x + 9) = x^2 - 81
This is an upward turning parabola.....we will have no max.....but we will have a minimum (at the vertex )
The vertex is at (0, -81)....so this is the minimum = -81
Then this must imply that x = 0 because 0^2 - 81 = -81
So
( x - 9) = -9 ⇒ the lesser integer
And
(x + 9) = 9 ⇒ the greater integer