Answer:
B (-6,6)
Step-by-step explanation:
For two points (a,b) and (c,d), the co-ordinate of the midpoint of the line joining the points is ((a+c)/2 , (b+d)/2)
Let, co-ordnate of B is (x,y)
So, (4+x)/2 = -1 ; (8+y)/2 = 7
=> x = -6 ; => y = 6
[If there's any mistake, please inform me]
