40282

(a/2 x a) + (a/2 x b)+(b/2 x b) + (a/2 x b)= the area of the yellow space

1 - (a/2 x a) + (a/2 x b)+(b/2 x b) + (a/2 x b)= area of the red quadrilateral
Let us now substitute the values of a and b: a=6 b=18
18+54+162+54=288
area of yellow space=288
area of red space=(a+b)squared-288
therefore 576-288=288