Intermediate Algebra (6th Edition)

$c=8$
The coefficient of the middle term of a perfect square trinomial is equal to twice the square root of the third term. Hence, to make the given expression, $x^2+cx+16$, a perfect square trinomial, then, \begin{array}{l} c=2\sqrt{16} \\\\ c=2\cdot4 \\\\ c=8 .\end{array}