In a non-magnetic dielectric a plane wave of frequency 80 MHz has a peak electric field of 24 V/m…

Step 1. The speed. $v = \dfrac{E_0}{B_0} = \dfrac{24\ \text{V/m}}{150 \times 10^{-9}\ \text{T}} = 1.60 \times 10^{8}\ \text{m/s}$.

Step 2. The index. $n = \dfrac{c}{v} = \dfrac{3.00 \times 10^{8}\ \text{m/s}}{1.60 \times 10^{8}\ \text{m/s}} = 1.88$.

Step 3. The permittivity. With $\mu_r = 1$, $\varepsilon_r = n^2 = 3.52$.

Step 4. The wavelength. $\lambda = \dfrac{v}{f} = \dfrac{1.60 \times 10^{8}\ \text{m/s}}{80 \times 10^{6}\ \text{Hz}} = 2.00\ \text{m}$.