I can comprehend the problem with your MWE. Seems to be caused by the different height of the second item so the
multicol package runs into trouble. I suggest to use the
inparaenum environment from the
paralist package.
Code: Select all
\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[neverdecrease]{paralist}
\pagestyle{empty}
\begin{document}
\begin{inparaenum}[a)]
\item $\displaystyle z(x;y)=x-y$\qquad
\item $\displaystyle z(x;y)=\frac{x}{y}$\qquad
\item $\displaystyle z(x;y)=x+y$
\end{inparaenum}
\end{document}
As you can see you have to add some horizontal space but finally you got a proper vertical alignment. Another idea could be to "hide" the height of the fraction to make the compiler neglect it.
Code: Select all
\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{multicol}
\usepackage[neverdecrease]{paralist}
\pagestyle{empty}
\begin{document}
\begin{multicols}{3}
\begin{enumerate}[a)]
\item $\displaystyle z(x;y)=x-y$
\item $\displaystyle z(x;y)=\smash{\frac{x}{y}}$
\item $\displaystyle z(x;y)=x+y$
\end{enumerate}
\end{multicols}
\end{document}
This aligns the items vertically in a proper way.
Best regards and welcome to the board
Thorsten