Two plates of a conductivity cell each have an area of 2.5 cm²…

Step 1. The cell constant. $G^* = \dfrac{l}{A} = \dfrac{7.5\ \text{cm}}{2.5\ \text{cm}^2} = 3.0\ \text{cm}^{-1}$. One $\text{cm}^{-1}$ is $100\ \text{m}^{-1}$, so $G^* = 300\ \text{m}^{-1}$.

Step 2. The conductivity. $\kappa = \dfrac{G^*}{R} = \dfrac{300\ \text{m}^{-1}}{1200\ \Omega} = 0.25\ \text{S/m}$.

Step 3. In S/cm. $0.25\ \text{S/m} \div 100 = 0.0025\ \text{S/cm}$.