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{lengths} is for questions dealing with measurements in TeX. If your question is about the spacing between document elements, use {spacing} instead.

The calc package is typically used in length calculations, allowing users to do

\newlength{\mylen}
\setlength{\mylen}{2pt-3em+5pc}% 29.14992pt under 11pt font

similar to plain TeX's \dimexpr 2pt-3em+5pc\relax. In pure LaTeX, the equivalent would require

\newlength{\mylen}
\setlength{\mylen}{2pt}
\addtolength{\mylen}{-3em}
\addtolength{\mylen}{5pc}
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