Given: Frequency $f_1=6\mathrm{MHz}$

Frequency $f_2=10\mathrm{MHz}$

Wavelength can be calculated using $\lambda =\frac{c}{f}$.

$\Rightarrow {\lambda }_1=\frac{c}{f_1}=\frac{3\times {10}^8}{6\times {10}^6}=50 \mathrm{m}$

$\Rightarrow {\lambda }_2=\frac{c}{f_2}=\frac{3\times {10}^8}{10\times {10}^6}=30 \mathrm{m}$

Wavelength bandwidth is the difference of upper lower wavelengths $={\lambda }_1-{\lambda }_2=20 \mathrm{m}$