You can put this solution on YOUR website! The equation of the locus of points that are equidistant from the points (a+b,a-b) and (a-b,a+b) is
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x = y.
And ignore the writing by "josgarithmetic": it is not relevant.
The points equidistant from those two variable described points would be a LINE. This line will intersect the midpoint of the two described points and will have slope negative reciprocal of that of the two described points. Now, this is why "find slope and midpoint of those...".