Solución
$$\mathop {\lim }\limits_{x \to 6} 2f\left( x \right)g\left( x \right) = 2\left( {\mathop {\lim }\limits_{x \to 6} f\left( x \right)} \right)\left( {\mathop {\lim }\limits_{x \to 6} g\left( x \right)} \right) = 72$$