The LaTeX command for left right brackets is \left[ \right] (auto-sized square brackets).
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Write \left[ \right] 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.
Write \left[ \right]; for delimiters that grow with their content wrap them in \left and \right, or pick a fixed size with \bigl and \bigr, often the better choice.
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\left( \right)\left\{ \right\}\langle \rangle\lfloor \rfloor\lceil \rceil\| x \|| x |\lvert x \rvert\lVert x \rVert\left| \right|Delimiters in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \left[ \right] in math mode; it needs no extra package. For example, $\left[ \right]$.
None: \left[ \right] is built into LaTeX, so no extra package is required.