Answer :

midpoint formula - [tex]\left(\dfrac{x_1+x_2}{2},\dfrac{y_1+y_2}{2}\right)[/tex]
[tex] \left(\dfrac{3x+5+x-1}{2},\dfrac{3y-y}{2}\right)=\\ \left(\dfrac{4x+4}{2},\dfrac{2y}{2}\right)=\\ (2x+2,y)[/tex]