The pattern for completing the square is:
ax² + bx + (b/2)² = c + (b/2)²
But in this quadratic equation, there is no bx, or b = 0:
x² = 16 or x² - 16 = 0
Therefore, use extracting the roots for the above example.
If we use completing the square in the example, it will appear like this:
x² + (0/2)² = 16 + (0/2)²
(x-0)² = 16 or (x)(x) = 16 ⇒What's the point, right?
While you may also use completing the square, it is not the appropriate one.
The only method that is applicable to any quadratic equation is quadratic formula.