The elevation in boiling point for 1 molal solution of non-volatile...

We know that

${\Delta T}_{b} = {iK}_{b}{m}_{1}{\Delta T}_{f} = {iK}_{f}{m}_{2}$

And for non-volatile solute $i = 1$.

$\dfrac{{\Delta T}_{b}}{{\Delta T}_{f}} = \dfrac{{K}_{b} \times 1}{{K}_{f} \times 2} ⇒ \dfrac{3}{6} = \dfrac{1}{2} = \dfrac{{K}_{b}}{{K}_{f}} \times \dfrac{1}{2}$

$\dfrac{{K}_{b}}{{K}_{f}} = \dfrac{1}{1} ⇒ X = 1$