Question 704144
Opposite sides are parallel, and
the diagonals are perpendicular bisectors of each other.
You can prove it is a rhombus if you prove that
the diagonals are perpendicular bisectors of each other.
 
It is easy to see that EG is vertical, part of the line {{{x=0}}},
and that FH is horizontal, part of the line {{{y=2}}},
and that the diagonals intersect at point (0,2).
 
What remains to prove is that (0,2) is the midpoint of EG, and the midpoint of FH.
You do remember that the coordinates of the midpoint of a segment are the averages of the coordinates of the two end points, right?