If you graph this line you get a vertical line that runs between 3 and 11. It looks like this.
the mid point is on the vertical line with an x value of 5 and lies half way between the y values of 3 and 11.
3 minus 11 gives us the distance of 8. Half of that gives us
1/2(8)=4
starting point of the line segment is 5,3 so the midpoint is 5, (3+4) or 5,7