Answer
9
Work Step by Step
The third term, $c,$ of a perfect square trinomial, $ax^2+bx+c,$ is given by
\begin{align*}
c=\left( \dfrac{b}{2a} \right)^2
.\end{align*}
Given the expression, $
x^2-6x
,$ the third term $c$ that will make this a perfect square trinomial is
\begin{align*}
c&=\left( \dfrac{-6}{2(1)} \right)^2
\\\\&=
\left( \dfrac{-6}{2} \right)^2
\\\\&=
\left( -3 \right)^2
\\&=
9
.\end{align*}
Hence, $
9
$ should be added to complete the square of the given expression.
