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\documentclass{article}
\usepackage{amsmath}
\usepackage[inline]{enumitem}
\usepackage{cancel}
\usepackage[T1]{fontenc}
\usepackage{mathtools}
\usepackage[version=3]{mhchem}
\setlength{\parskip}{0in}
\setlength{\parindent}{0in}
\everymath{\displaystyle}
\begin{document}
\begin{enumerate}
\item \ce{$\underset{\clap{\begin{minipage}[t]{2cm}\vspace{0.5\baselineskip}\centering\scriptsize\textsf{27 g/mol \\ 0.0306 mol}\end{minipage}}}{\ce{2Al}}$ + $\underset{\clap{\begin{minipage}[t]{2cm}\centering\scriptsize\textsf{4.5 g \\ 98.1 g/mol \\ 0.0459 mol}\end{minipage}}}{\ce{Zn(OH)2}}$ -> Al2(SO4)3 + 3H2}
\begin{align*}
n_{\ce{H2SO4}}=\dfrac{\text{4.5 \cancel{g}}}{\text{98.1 \cancel{g}/mol}} &\qquad \begin{aligned}[t]n_{\ce{Al}}&=\smash{\text{0.0459 mol \cancel{\ce{H2SO4}}}\times\dfrac{\text{2 mol Al}}{\text{3 mol \cancel{\ce{H2SO4}}}}} \\ &= \text{0.0306 mol}\end{aligned} \\
&\qquad\begin{aligned}[t]m_{\ce{Al}}&=\text{0.0306 mol $\times$ 27 g/mol} \\ &=\text{0.83 g}\end{aligned}
\end{align*}
\begin{enumerate*}[label=\alph*)]
\item \ce{H2O}
\end{enumerate*}
\end{enumerate}
\end{document}