A(7,-10), B(-3, 4) and C(8,-9)
Find the centroid of the triangle with the given vertices

Take the average of the three points. Add their coordinates together:

(7, -10) + (-3, 4) + (8, -9) = (7 - 3 + 8, -10 + 4 - 9) = (12, -15)

Divide each coordinate by 3:

(12, -15) / 3 = (4, -5)

the answer is (4,3)

since the rule for r0, 90º is (-y,x) and it was already applied to form the image, all you have to do is reverse the answer using this formula.