#### Answer

$c=9$

#### Work Step by Step

The third term of a perfect square trinomial is equal to the square of half of the coefficient of the middle term. Hence, to make the given expression, $
x^2+6x+c
$, a perfect square trinomial, then,
\begin{array}{l}
c=\left( \dfrac{6}{2} \right)^2
\\\\
c=\left( 3 \right)^2
\\\\
c=9
.\end{array}