Question 502323
Let pint B  be (1,2)
Mid point A (-3,6)

Difference in x coordinate between point B and A is 4
Difference in y coordinate between point B and A is 4

Since A is midpoint, the distance between the point c(x1,y1) will same as the distance between point B and A.

Hence x1 = -7 (4 units away from x cordinate of point B)
y1 = 10

Poinnt C = (-7,10)