check the picture below
in a parallelogra, the diagonals "bisect" each other, thus each one cuts the other in 2 equal halves.
that simply means in this case, where they intersect, is the midpoint of either diagonal... so let's use the midpoint of XZ then
[tex]\bf \textit{middle point of 2 points }\\ \quad \\
\begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
X&({{ 8}}\quad ,&{{ 7}})\quad
% (c,d)
Z&({{ -3}}\quad ,&{{ -2}})
\end{array}\qquad
% coordinates of midpoint
\left(\cfrac{{{ x_2}} + {{ x_1}}}{2}\quad ,\quad \cfrac{{{ y_2}} + {{ y_1}}}{2} \right)
\\\\\\
\left(\cfrac{{{ -3}} + {{ 8}}}{2}\quad ,\quad \cfrac{{{ -2}} + {{ 7}}}{2} \right)[/tex]
