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We want to complete the square for the given expression. To do so, we will first identify $b.$ In a quadratic trinomial, $b$ is the linear coefficient, which is the number that is multiplied by the $x\text{-}$variable. $\begin{gathered} x^2-4x+c \end{gathered}$ Now, we will calculate $\left(\frac{b}{2}\right)^2.$ Since we have that $b=\text{-}4,$ we can calculate the value of $\left(\frac{b}{2}\right)^2$ by substituting $\text{-}4$ for $b.$
$\left(\dfrac{b}{2}\right)^2$
$\left(\dfrac{{\color{#0000FF}{\text{-}4}}}{2}\right)^2$
$(\text{-}2)^2$
$4$
The number that completes the square is $c=4.$ $\begin{gathered} x^2-4x+4 \end{gathered}$