AEB is a right isoceles triangle. It means that it is half a square. So, AB is the diagonal of the square with side AE.
The side and diagonal of a square are linked by the following formula:
[tex] d = l\sqrt{2} [/tex]
which means
[tex] 18\sqrt{2} = l\sqrt{2} [/tex]
and this AE=EB=18.
The area of a parallelogram is given by the multiplication of the base and the height, which in our case means
[tex] A = \overline{BD}\cdot\overline{EB} = 18\cdot 28 = 504 [/tex]
