Log

Relevant wiki: Solving Logarithmic Equations - Basic

$2 \log x = \log (12 - x)$

$\log x^{2} = \log (12 - x)$

$x^{2} = 12 - x$

$x^{2} + x - 12 = 0$

$(x - 3)(x + 4) = 0$

$x = 3$ or $x = -4$

Since $x > 0$ , so $x = \boxed{3}$