1. The problem is to simplify the expression $-\sqrt{36x^2}$ assuming $x$ is a positive real number. 2. Recall the property of square roots: $\sqrt{a^2} = |a|$. Since $x$ is positive, $|x| = x$. 3. Simplify inside the square root: $\sqrt{36x^2} = \sqrt{36} \cdot \sqrt{x^2} = 6x$. 4. Now apply the negative sign outside: $-\sqrt{36x^2} = -6x$. 5. Therefore, the simplified expression is $-6x$.