The LaTeX command for double prime (″) is x''.
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Write x'' inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
x'' is a math-mode symbol: in running text, wrap it in $...$. With unicode-math under XeLaTeX or LuaLaTeX, typing ″ in a formula gives the same symbol.
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Use x'' in math mode; it needs no extra package. For example, $x''$.
None: x'' is built into LaTeX, so no extra package is required.