Let the measure of side AB be x, then, the measue of side AE is given by
[tex]AE=\sqrt{9^2-x^2}[/tex].
Now, ABCD is a square of size x, thus the area of square ABCD is given by
[tex]Area=x^2[/tex]
Also, AEFG is a square of size [tex]\sqrt{9^2-x^2}[/tex], thus, the area of square AEFG is given by
[tex]Area=\left(\sqrt{9^2-x^2}\right)^2=9^2-x^2=81-x^2[/tex]
The sum of the areas of the two squares ABCD and AEFG is given by
[tex]x^2+81-x^2=81[/tex]
Therefore, the number of square units in the sum of the areas of the two squares ABCD and AEFG is 81 square units.