A trapezoid with legs MN = FD is an isosceles trapezoid.
An isosceles trapezoid has diagonals that are congruent, therefore: MF = ND.
A quadrilateral with diagonals congruent and perpendicular is a square.
In a square, the height coincides with the side.
The area of a square is given by A = s²
Therefore, the area of MNFD is h²