Find the area of the region colored yellow

The region colored yellow is composed for 4 congruent isosceles triangles. The base and height of each triangle is 3 and 2.5. The area of one triangle is $\dfrac{1}{2}(3)(2.5)=3.75$ . Thus, the area of the region colored yellow is $4 \times 3.75 = 15$ .

The yellow area is a part of rhombus. Using Pythagoras,the diagonal of the bigger square is 8√2, and the smaller one is 3√2. The yellow area = (8√2)( 3√2)/2 – 9 = 15.