What is the conjugate of the complex number 5 + 3i?

$5 - 3 i$
For any complex number $z = x + i y$ in rectangular form, the complex conjugate is given by $\overline{z} = x - i y$.
Note also that $z \overline{z} = \left({x}^{2} + {y}^{2}\right) + 0 i \in \mathbb{R}$