we know that
The formula to calculate the midpoint between two points is equal to
[tex]M(\frac{x1+x2}{2},\frac{y1+y2}{2})[/tex]In this problem we have
M(-3,9)
(x1,y1)=(3,-6)
(x2,y2)=?
substitute the given values in the formula
[tex](-3,9)=(\frac{3+x2}{2},\frac{-6+y2}{2})[/tex]Find the x1 coordinate
we have
-3=(3+x2)/2
3+x2=-6
x2=-6-3
x2=-9
Find the y2 coordinate
we have
9=(-6+y2)/2
y2=18+6
y2=24
therefore