The LaTeX command for ddot deriv (ẍ) is \ddot{x} (second time derivative).
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Write \ddot{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.
The physics and siunitx packages offer shortcuts for many formulas, but \ddot{x} works in plain LaTeX. With unicode-math under XeLaTeX or LuaLaTeX, typing ẍ in a formula gives the same symbol.
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\hbar\nabla\nabla^2\nabla \cdot\nabla \times\partial\dot{x}\langle \psi || \psi \rangle\langle \phi | \psi \rangle\rightleftharpoons\text{\AA}^\circ\mathrm{C}\inftyPhysics symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \ddot{x} in math mode; it needs no extra package. For example, $\ddot{x}$.
None: \ddot{x} is built into LaTeX, so no extra package is required.