#JavaScript (ES6), <s>&nbsp;21&nbsp;</s> 20 bytes

Takes input as `(a)(b)`.

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 a=>b=>[a.length,a]+b

[Try it online!](https://tio.run/##bcq7DYAgEADQ3jGoICobwCLG4vhrLpwBouNjaI31eyfcUG05rrZmcr4H1UFpo/QGEn2OLS2wz6ZbypXQS6TIA2fJIxITnD1U0DEhpk@Agcb@krEDB/UX "JavaScript (Node.js) – Try It Online")

Returns the length of \$a\$, followed by a comma, followed by the concatenation of \$a\$ and \$b\$.