Midpoint

A(3x+5x, 3y) and B(x-1, -y)

I hope this answer is correct!!

If there is a typo in the x co-ords of A
such that it should read:

A(3x+5, 3y) and B(x-1, -y).

then

Midpoint x
= (3x+5 + x-1) /2
= (4x+4)/2
= 2x+2

Midpoint y
= (3y + (-y)) /2
= (3y -y) /2
= 2y/2
=y

So, Midpoint AB = (2x+2, y)

However, if it wasn't a typo
then

Midpoint x
= (3x+5x + x-1) /2
= (8x + x-1) /2
= (9x-1)/2
= 4.5x - 0.5

Midpoint y
= (3y + (-y)) /2
= (3y -y) /2
= 2y/2
=y

So, Midpoint AB = (4.5x - 0.5, y)