Respuesta :
[tex]\bf ~~~~~~~~~~~~\textit{middle point of 2 points }
\\\\
J(\stackrel{x_1}{4}~,~\stackrel{y_1}{-15})\qquad
K(\stackrel{x_2}{x}~,~\stackrel{y_2}{y})
\qquad
% coordinates of midpoint
\left(\cfrac{ x_2 + x_1}{2}~~~ ,~~~ \cfrac{ y_2 + y_1}{2} \right)
\\\\\\
\left( \cfrac{x+4}{2}~~,~~\cfrac{y-15}{2} \right)\implies \stackrel{midpoint}{L(-1,8)}\implies
\begin{cases}
\cfrac{x+4}{2}=-1\\\\
x+4=-2\\
\boxed{x=-6}\\
------\\
\cfrac{y-15}{2}=8\\\\
y-15=16\\
\boxed{y=21}
\end{cases}[/tex]
