I suggest you use \text instructions for the 2 strings that don't need line breaking and \parbox directives for the 2 strings that do. Both \text and \parbox typeset their arguments in text mode, which is precisely what you want.
Remark: As @barbarabeeton has noted in a comment, if there's any chance at all that this type of "wordy equation" will occur in a setting that employs italics as the main font shape (such as the interior of a theorem-like environment), you should use \textup or \textrm instead of \text in order to assure that upright rather than italic characters are used.
For the following example code, I chose the widths of the \parboxes after a minimal amount of trial and error.

\documentclass{article}
\usepackage{amsmath} % for '\text' macro
\begin{document}
[
\text{Maximum credit} =
\frac{\parbox{4cm}{\centering Net income from sources
without the United States\strut}}{%
\text{Total net income}}
\times
\parbox{2.75cm}{\centering United States tax on total income}
]
\end{document}
\text{…}instead of\rm{…}– Luis Turcio Nov 18 '21 at 00:28\rmshould never be used in latex, it has not been defined by default for almost 30 years, if you do use a class that defines it for compatibility with old documents then the syntax is{\rm Maximum}not\rm{Maximum}but\textrm{Maximum credit}does what you want. – David Carlisle Nov 18 '21 at 08:11