Answer
$x^2-4x+4
\Rightarrow
(x-2)^2
$
Work Step by Step
The third term of a perfect square trinomial is equal to the square of half the coefficient of the middle term. Hence, to complete the square of the given expression $
x^2-4x
,$ the third term must be
\begin{array}{l}\require{cancel}\left( \dfrac{-4}{2}
\right)^2\\\\=\left(
-2
\right)^2\\\\=
4
.\end{array}
Using $a^2\pm2ab+b^2=(a\pm b)^2$, then
\begin{array}{l}\require{cancel}
x^2-4x+4
\Rightarrow
(x-2)^2
.\end{array}