In general, given two points, the midpoint between those two points is given by the formula below
[tex]\begin{gathered} (x_1,y_1),(x_2,y_2) \\ \Rightarrow midpoint=(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2}) \end{gathered}[/tex]Therefore, in our case, if B=(x,y),
[tex]\begin{gathered} (4,-2)=(\frac{3+x}{2},\frac{-6+y}{2}) \\ \Rightarrow4=\frac{3+x}{2} \\ and \\ -2=\frac{-6+y}{2} \\ \Rightarrow8=3+x \\ and \\ -4=-6+y \\ \Rightarrow x=5 \\ and \\ y=2 \end{gathered}[/tex][tex]\Rightarrow B=(5,2)[/tex]