% inv op worksheet template
\documentclass[varwidth]{standalone}
\usepackage{amsmath}
\usepackage[normalem]{ulem} % [normalem] prevents the package from changing the default behavior of `\\emph` to underline.
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{align*}

\begin{document}
    \begin{varwidth}{0.25\paperwidth}
    \begin{align*}
    x - 4 &= 9\\
    x - 4 + \dotuline{\hspace{4mm}} &= 9 + \dotuline{\hspace{4mm}}\\
    x &= \dotuline{\hspace{4mm}}\\
\end{align*}
\end{varwidth}

\end{document}