How to align objects on different lines horizontally?

Question

I want to start an exercise

1. by a number (I.1 or XII.32)
2. then some space
3. and then the beginning of the exercise so that all the beginnings of the exercises (point 3. above) are horizontally aligned, as in the following picture.

Exercises belong to different paragraphs and may have several lines and centered formulas.

How can I align the beginning of the exercises of a same page?

Answer 1

Use \hbox to2cm{...} ...

Use \leavevmode to leave vertical mode and switch to horizontal mode.

\documentclass{article}

\begin{document}
\leavevmode\hbox to2cm{\textbf{I.1}} First exercise 

\leavevmode\hbox to2cm{\textbf{XII.13}} Last exercise
\end{document}

Answer 2

I'd suggest a list rather than setting each exercise manually. You can use enumitem to format the list setting globally (like I did below by creating an exercises list):

\documentclass{article}

\usepackage{enumitem,lipsum}

\newlist{exercises}{enumerate}{1}
\setlist[exercises]{
  labelwidth=2cm,
  font=\bfseries,
  align=left
}

\begin{document}

\begin{exercises}
  \item[I.1]
    First exercise 

  \item[IX.78]
    \lipsum[1]
    
  \item[XII.13]
    Last exercise
\end{exercises}

\end{document}