The midpoint of cd is -9,-10, endpoint c has coordinates of -1,9 what is the endpoint of d?

From c to the midpoint the coordinates have changed from (-1;9) to (-9;-10). So the difference is (-9-(-1); -10-(9)) that equals (-8;-19). And you should add (-8;-19) to the coordinates of the middle point (-9;-10). So we get (-9+(-8); -10+(-19)) that is (-17;-29). This is the endpoint of d. d (-17;-29).