We can use the following identity:
(Xm, Ym) = X1+X2/2, Y1+Y2/2, where Xm and Ym are the midpoints of X and Y.
So, let's use it.
For question 11, X1 is -3, Y1 is 9, X2 is 3, and Y2 is -7.
Plugging in the points, we get:
-3 + 3 = 0 / 2 = 0 for x, and 9 + (-7) = 2 / 2 = 1 for y. Thus the midpoint is (0,1).
For question 12, X1 is -9, Y1 is -4, X2 is 1, and Y2 is 6. Plugging in the values we get:
-9 + 1 = -8 / 2 = -4 for x, and -4 + 6 = 2 / 2 = 1 for y. So the midpoint is (-4,1)