[tex]\text{The formula of a midpoint:}\\\\\left(\dfrac{x_1+x_2}{2};\ \dfrac{y_1+y_2}{2}\right)[/tex]
[tex]\text{We have}\\\\\text{endpoint}\ (-9,\ -11)\to x_1=-9,\ y_1=-11\\\\\text{midpoint}\ (-18,\ -12)\\\\\text{Therefore we have the equations:}\\\\\dfrac{-9+x_2}{2}=-18\qquad\text{multiply both sides by 2}\\\\-9+x_2=-36\qquad\text{add 9 to both sides}\\\\\boxed{x_2=-27}\\\\\dfrac{-11+y_2}{2}=-12\qquad\text{multiply both sides by 2}\\\\-11+y_2=-24\qquad\text{add 11 to both sides}\\\\\boxed{y_2=-13}\\\\Answer:\ \boxed{(-27,\ -13)}[/tex]
