Answer
$\dfrac{25}{4}$
Work Step by Step
To find the third term that will make the given expression, $
x^2-5x
,$ a perfect square, divide the coefficient of the middle term by $2$ and then square the result. That is,
\begin{array}{l}\require{cancel}
\left( \dfrac{-5}{2} \right)^2
\\=
\dfrac{25}{4}
.\end{array}
Hence, the third term of the given expression should be $
\dfrac{25}{4}
$ so that it becomes a perfect square.
