Hi

Your work is correct

Let I be the midpoint of [AB], J the midpoint of [BC], K the midpoint of [CD] and L the midpoint of [DA]

From the data we have I(2,5) ; J(3,2) ; K((a+5)/2,(b+3)/2) ; L((a+3)/2,(b+9)/2)

Using vectors

[IJ] and [JK] being two sides of a square, they are perpendicular

The dot product of

and

is equal to 0

[1]

The lengths of [IJ] and [JK] are equal

Using [1]

b-1=2 => b=3 => a=7 => D(7,3)

b-1=-2 => b=-1 => a=-5 => D'(-5,-1)

There are 2 solutions depending if you consider ABCD' as a quadrilateral (probably not)