Calculus: Early Transcendentals 8th Edition

$\lim\limits_{x \to 4}\dfrac{x^{2}+3x}{x^{2}-x-12}=\infty$
Apply direct substitution first: $\lim\limits_{x \to 4}\dfrac{x^{2}+3x}{x^{2}-x-12}=\dfrac{4^{2}+3(4)}{4^{2}-4-12}=\dfrac{28}{0}=\infty$