[tex]\bf \begin{cases}
z1=9-9i\implies &(9~,~-9)\\
z2=10-9i\implies &(10~,~-9)
\end{cases}[/tex]
[tex]\bf ~~~~~~~~~~~~\textit{middle point of 2 points }\\\\
\begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&&(~ 9 &,& -9~)
% (c,d)
&&(~ 10 &,& -9~)
\end{array}\qquad
% coordinates of midpoint
\left(\cfrac{ x_2 + x_1}{2}\quad ,\quad \cfrac{ y_2 + y_1}{2} \right)
\\\\\\
\left( \cfrac{10+9}{2}~~,~~\cfrac{-9-9}{2} \right)\implies \left( \cfrac{19}{2}~~~,~~\cfrac{-18}{2} \right)
\\\\\\
\left( \cfrac{19}{2}~~~,~~-9 \right)\implies \left( 9\frac{1}{2}~~~,~~-9 \right)[/tex]
