Hi Maia,
a+b=10
so
b=10-a
ab=504
a(10-a)=504
-a^2+10a-504=0
a^2-10a+504=0
$${{\mathtt{a}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{\mathtt{10}}{\mathtt{\,\times\,}}{\mathtt{a}}{\mathtt{\,\small\textbf+\,}}{\mathtt{504}} = {\mathtt{0}} \Rightarrow \left\{ \begin{array}{l}{\mathtt{a}} = {\mathtt{5}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{479}}}}{\mathtt{\,\times\,}}{i}\\
{\mathtt{a}} = {\sqrt{{\mathtt{479}}}}{\mathtt{\,\times\,}}{i}{\mathtt{\,\small\textbf+\,}}{\mathtt{5}}\\
\end{array} \right\} \Rightarrow \left\{ \begin{array}{l}{\mathtt{a}} = {\mathtt{5}}{\mathtt{\,-\,}}{\mathtt{21.886\: \!068\: \!628\: \!239\: \!289\: \!2}}{i}\\
{\mathtt{a}} = {\mathtt{5}}{\mathtt{\,\small\textbf+\,}}{\mathtt{21.886\: \!068\: \!628\: \!239\: \!289\: \!2}}{i}\\
\end{array} \right\}$$
There are no real solutions to this .