From this diagram, we are given that the radius of the circle is 2cm. On the diagram I have drawn, I denoted 3 points, A, B and C, of which A and C are touching the circumference of the circle.
Point A to point B is the radius of the circle (2cm), which is also the length of a right-angled triangle as drawn on the paper. The other diagonal line would similarly be 2cm.
From this, we can find the area of 1 triangle 1/2 x base x height, and get the answer of 2. To find the full shaded area, you just need to multiply that value by the amount of triangles, which is 4, and you will receive the final answer 8cm.