The locus of a point would be:View attachment 13103
My answer that I formulated was:
The locus is Point D and B are equidistant to A and C because all sides of a square are congruent & the midpoint of the diagonal of A and C equidistant to both A and C.
It's hard to untangle your grammar to see how close you are to the right idea. Points D and B are on the locus, as is the midpoint of the diagonal, but they are not the entire locus. Where will all such points be? (For example, "the line determined by ...")View attachment 13103
My answer that I formulated was:
The locus is Point D and B are equidistant to A and C because all sides of a square are congruent & the midpoint of the diagonal of A and C equidistant to both A and C.