Acid - Base Equilibrium
You have an error somewhere in your math.
Assuming you were supposed to interpolate $K_C$ from both experiment 1 and 2, you would do the following:
Experiment1 yields $K_C=3.31$. Experiement two yields $K_C=3.40$. Interpolation yields $K_C=3.35$.
This would give you $$B=\sqrt{\frac{C^2}{A\cdot K_C}}=\sqrt{\frac{1.49^2}{1.4\cdot 3.35}}=0.69$$