The formula of the midpoint of MO:
[tex]N\left(\dfrac{x_M+x_O}{2};\ \dfrac{y_M+y_O}{2}\right)[/tex]
We have N(15; 12) and M(-4; 9), O(x; y). Substitute:
[tex](15;\ 12)=\left(\dfrac{-4+x}{2};\ \dfrac{9+y}{2}\right)\\\\\dfrac{-4+x}{2}=15\ \ \ \ |\cdot2\\\\-4+x=30\ \ \ |+4\\\\x=34\\\\\dfrac{9+y}{2}=12\ \ \ \ |\cdot2\\\\9+y=24\ \ \ |-9\\\\y=15[/tex]
Your answer is [tex]N(34;\ 15)[/tex]
