define $d | a$, read "d divides a"
$\left[\ \exists(a \in \mathbb{Z}) \land \exists(d \in \mathbb{N}) \land (d | a)\ \right] \Longleftrightarrow \left[\ \exists(a \in \mathbb{Z}) \land \exists(d \in \mathbb{N}) \land \exists(k \in \mathbb{Z}) \land (a = dk)\ \right]$
Suppose $a$ is an integer and suppose $d$ is a natural number. We can say that $d$ divides $a$ if and only if there is an integer $k$ such that $a = dk$.
(3)