Answer:
-3
Step-by-step explanation:
When the leading coefficient is 1, as it is here, the value that goes in the blank is half the x-coefficient. -6/2 = -3.
-3 goes in the blank.
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The value of 'c' will be the square of this: (-3)² = 9.
x² -6x +9 = (x -3)²
