Solución
$$x\mathop {\lim }\limits_{x \to - {3^ - }} \left( {f\left( x \right) - 3g\left( x \right)} \right) = \mathop {\lim }\limits_{x \to - {3^ - }} f\left( x \right) - 3\mathop {\lim }\limits_{x \to - {3^ - }} g\left( x \right) = 0 + 6 = 6$$