The midpoint of AB is M(-1,7). If the coordinates of A are (4,8), what are the
coordinates of B?

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]

actually what is a question i don't understand