The point (-11,34) has the coordinates x = -11 and y = 34
The point (47,0) has the coordinates x = 47 and y = 0
Let's focus on just the x coordinates of each point for now. Those values are -11 and 47. Add them up to get -11+47 = 36. Then divide by two to end up with 36/2 = 18
The x coordinate of the midpoint is x = 18
Repeat those steps for the y coordinates. First add them up: 34+0 = 34. Then divide by two: 34/2 = 17
The y coordinate of the midpoint is y = 17
Therefore the midpoint is located at (x,y) = (18,17)
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side note:
If you want, you can use the formulas
xm = (x1+x2)/2
ym = (y1+y2)/2
where (x1,y1) and (x2,y1) are the two endpoints and (xm, ym) is the midpoint. Though this method may be a bit confusing.