Explanation:
Let x be the first integer
Let y be the 2nd integer
Let z be the 3rd integer
Given
Also given, x = 7, y = 8, z = 9, substitute x,y and z into equation.
Therefore,
x+y+z = 3y for x = 7, y = 8, z = 9.
Now lets take x = 3, y = 4, z = 5.
As you can see , x+y+z = 3y is true for any 3 consecutive numbers.