A chord of a circle of radius 18 cm subtends an angle of 120° at the centre…

Step 1. The sector. $\dfrac{1}{3} \times \pi \times 18^2 = 108\pi\ \text{cm}^2$. With $\pi = 3.14$ this is $339.12\ \text{cm}^2$.

Step 2. The triangle. $\dfrac{1}{2} \times 18^2 \times \dfrac{\sqrt{3}}{2} = 81\sqrt{3}\ \text{cm}^2$. With $\sqrt{3} = 1.73$ this is $140.13\ \text{cm}^2$.

Step 3. The segment. $339.12 - 140.13 = 198.99\ \text{cm}^2$.