Answer
number to be added: $4$
factored form: $(x-2)^2$
Work Step by Step
RECALL:
To complete the square of an expression in the form $x^2-bx$, $(\frac{b}{a})^2$ must be added.
The factored form of the result is $(x-\frac{b}{2})^2$.
Thus, in the given expression add $(\frac{4}{2})^2=2^2=4$ to obtain:
$x^2 -4x + 4$
Writing the square in factored form gives:
$x^2-4x+4 = (x-2)^2$
