Using the formula

x₁= 10 y₁=12 Xm=2 Ym = 8
x₂ = ? y₂=?
Substituting and solving for x₂


Multiply both-side of the equation by 2
 4 = 10 + x₂
subtract 10 from both-side of the equation
 -6 = x₂
 x₂= -6
Similarly, substituting and solving for y₂


Multiply both-side of the equation by 2
 16 = 12 + y₂
Subtract 12 from both-side of the equation
 4 = y₂
 y₂= 2
Hence; coordinates of B are;
B(x,y) = ( -6, 2)