\documentclass[12pt]{exam}
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\begin{document}

\begin{questions}


\question Let $f(x) = \frac{3x+1}{x + \sqrt{4x^2+5}}$.

\begin{parts}

\part Compute $\displaystyle\lim_{x \to \infty} f(x)$.

\part Compute $\displaystyle\lim_{x \to -\infty} f(x)$. [Warning: $\sqrt{(-10)^2}=10$, not $-10$.]

\part Design a continuous function $g(x)$ so that $\displaystyle\lim_{x \to \infty} g(x) = -2$ and $\displaystyle\lim_{x \to -\infty} g(x) = 0$.

\part How many different horizontal asymptotes can a function have?

\end{parts}

\end{questions}

\end{document}