(-1,5) and (-1,-3) are the ends of a vertical line. Since (3,1) is not
at the same y-coordinate as either of those, the vertical line must be a
diagonal. It has length=8.
Since the diagonal has length 8, its sides have length 4√2, giving an area of 32.
Knowing that the other diagonal is horizontal and has length=8, it
should be easy to plot the missing vertex and draw the square.
For what it's worth, the diagonal is a line segment joining two non-consecutive vertices, not sides.
LOL i wished i have explained well. *5th grader here*