I cannot make out the coordinates of A, but let those coordinates be (x,y).
Because this is isosceles the two halves of the base will be equal which means the coordinates of A will be (-x,y) because it is just on the opposite side of the y axis. Thus the y coordinate is the same and the x coordinate is the negative of the x from A...
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The midpoint of any segment is just the average of the coordinates of the endpoints so:
((3+7)/2, (5-1)/2)
(10/2, 4/2)
(5, 2)
