We use Bretschneider's formula:
If a, b, c, and d are the sides of a convex quadrilateral, and s is the
semiperimeter, 
and 
and 
are two opposite angles,
then the area A of the quadrilateral is given by
First we calculate s with a=10, b=5, c=14.14, d=15,
Then using that and 
,
A = 91.89795801035699017470925314054801987941948129813875441889863... cm².
Edwin
