Your answer would be A (-8,20). Why because in order to get a mid point you need the equation which is (x + x / 2 , y + y / 2) and if you plug in the numbers (-8,20) and (0,-10) you will end up with the midpoint which is (-4,5), Your welcome.
Mid point formula says M = X1+X2/2 ; Y1+Y2/2 X1+0/2=-4 Y1+(-10)/2=5 X1=-4x2 Y1-10=5x2 X1=-8 Y1=10+10=20 That means your answer is A- point R is (-8:20) Hope this helped