Assuming you mean the dark area, we can derive it by subtracting the area of the inner triangle from the area of the outer rectangle.
They are both pretty straightforward to compute: the outer area is given by the product of the dimensions:
[tex] A_{rect} = 5\cdot 6 = 30 [/tex]
The area of the triangle is half the product of base and height:
[tex] A_{tri} = \dfrac{3\cdot 3}{2} = \dfrac{9}{2} [/tex]
So, the dark area is given by
[tex] 30-\dfrac{9}{2} = \dfrac{60}{2}-\dfrac{9}{2} = \dfrac{60-9}{2} = \dfrac{51}{2} [/tex]
