Answer:
9
Explanation:
The trinomial square have the following form: 
(a - b)² 
Expanding this, the result is: 
a² -2ab +b²
So, let's put x in place of a and solve it:
x² -2•x•b + b²
Now, we know that initially -2•x•b was -6x in the given trinomial square, so let's find the value of b using an equation: 
-2x•b = -6x divide both sides by -2x:
b = -3 
So -3 complete our trinomial square, and it is: 
(x -3)² 
Expanding: 
x² -6x +9