Question

ABCD is a square . if the coordinates of three of its vertices are A(-1,2a), B(a,2a), C(a,0), find the coordinates of D.

BC is obviously 2a (it's a vertical line)
then AB must be 2a (notice AB is horizontal)

so BC = AC because it is a square
2a = a+1
a=1

Now you can find the coordinates of A, B, and C, it should be easy for you to find D.

Answer 1

D(-1,0)