Answer
4
Work Step by Step
Using the rules of logarithms to solve the equation, we find:
$$\log _4\left(\left(x+12\right)x\right)=3 \\ \log _4\left(\left(x+12\right)x\right)=\log _4\left(64\right) \\ \left(x+12\right)x=64 \\ x=4,\:x=-16$$
Checking for extraneous solutions, we see that only $x=4$ is valid.