Combine the like terms under the square root.

$$\sqrt{36x^2} = \sqrt{36} \cdot \sqrt{x^2} = 6|x|$$

The absolute value is used because $\sqrt{x^2}$ is $|x|$. If $x$ is assumed to be non-negative, then it simplifies to $6x$. Assuming $x \ge 0$:

$$\sqrt{36x^2} = 6x$$

The final answer is $\boxed{\text{6x}}$

$$\sqrt{25a^4b^6} = \sqrt{25} \cdot \sqrt{a^4} \cdot \sqrt{b^6} = 5 \cdot a^{4/2} \cdot b^{6/2} = 5a^2b^3$$

The final answer is \boxed{\text{5a^2b^3}}

$$\sqrt{64x^2} = \sqrt{64} \cdot \sqrt{x^2} = 8|x|$$

Assuming $x \ge 0$:

$$\sqrt{64x^2} = 8x$$

$$8x - 3x^2$$

The final answer is \boxed{\text{8x - 3x^2}}