Chapter 5 - Section 5.7 - Factoring by Special Products - Exercise Set - Page 310: 88

$c=12$

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+36 $, a perfect square trinomial, then, \begin{array}{l} c=2\sqrt{36} \\\\ c=2\cdot6 \\\\ c=12 .\end{array}