Frequency of oscillation is proportional to

Let mass is displaced towards left by $x$ then force on $\operatorname{mass}=-\mathrm{kx}-2 \mathrm{kx}=-3 \mathrm{kx}$ [negative sign is taken because force is opposite to the direction of motion] $$\begin{array}{l} \Rightarrow \mathrm{F}=-3 \mathrm{kx}=-\mathrm{m} \omega^{2} \mathrm{x} \Rightarrow \omega=\sqrt{\frac{3 \mathrm{k}}{\mathrm{m}}} \\ \mathrm{f}=\frac{\omega}{2 \pi}=\frac{1}{2 \pi} \sqrt{\frac{3 \mathrm{k}}{\mathrm{m}}} \end{array}$$Thus it is propotional to \sqrt{3 \mathrm{k} / \mathrm{m}}$