Solución
$$\mathop {\lim }\limits_{x \to 6} \left[ {\left( {x + 1} \right)f\left( x \right)} \right] = \left( {\mathop {\lim }\limits_{x \to 6} \left( {x + 1} \right)} \right)\left( {\mathop {\lim }\limits_{x \to 6} f\left( x \right)} \right) = 28$$