You can also use \hfill in case you want the parts between & & to be aligned to the left.

Another command to align things is \begin{gathered} \end{gathered} :

$\displaystyle \begin{gathered}

p \text{ is a prime.} \hfill \\

\text{In } \mathbb{Z}/p \mathbb{Z},~ \forall \overline{n},~ \overline{n}^p-\overline{n}= \prod_{k=0}^{n-1} (\overline{n}-k) \hfill \end{gathered}$

Code:

\begin{gathered}
p \text{ is a prime.} \hfill \\
\text{In } \mathbb{Z}/p \mathbb{Z},~ \forall \overline{n},~ \overline{n}^p-\overline{n}= \prod_{k=0}^{n-1} (\overline{n}-k) \hfill \end{gathered}

$\displaystyle \begin{gathered}

p \text{ is a prime.} \\

\text{In } \mathbb{Z}/p \mathbb{Z},~ \forall \overline{n},~ \overline{n}^p-\overline{n}= \prod_{k=0}^{n-1} (\overline{n}-k) \end{gathered}$

Code:

\begin{gathered}
p \text{ is a prime.} \hfill \\
\text{In } \mathbb{Z}/p \mathbb{Z},~ \forall \overline{n},~ \overline{n}^p-\overline{n}= \prod_{k=0}^{n-1} (\overline{n}-k) \end{gathered}

(without \hfill)