Solución

$$\mathop {\lim }\limits_{x \to 4} {{{x^2} - 16} \over {x - 4}} = {{16 - 16} \over {4 - 4}} = {0 \over 0}$$

Así, $$\mathop {\lim }\limits_{x \to 4} {{{x^2} - 16} \over {x - 4}} = \mathop {\lim }\limits_{t \to 4} {{\left( {x + 4} \right)\left( {x - 4} \right)} \over {x - 4}} = 8$$