If you draw a diagonal from upper left to lower right, you divide the figure into two triangles. One is a 3-4-5 right triangle (with area = (1/2)*3*4 = 6). The other is a 5-5-4 isosceles triangle.
The altitude of the isosceles triangle (upper left vertex to side 4) can be found from the Pythagorean theorem as
.. altitude = √(5^2 -2^2) = √21
So, the area of the isosceles triangle is
.. (1/2)*4*√21 = 2√21
The sum of the areas of the two triangles matches selection C.
