For a plane electromagnetic wave, the relationship between the electric field \( E \) and the magnetic field \( B \) is given by:
\[
B = \frac{E}{c},
\]
where \( c = 3 \times 10^8 \, \text{m/s} \) is the speed of light in a vacuum.
Given:
\[
E = 9.3 \, \text{V/m}, \quad f = 20 \, \text{MHz}.
\]
The wavelength \( \lambda \) of the wave can be found using the relationship:
\[
c = f \lambda \quad \Rightarrow \quad \lambda = \frac{c}{f}.
\]
Now, using the value \( E = 9.3 \, \text{V/m} \), the magnetic field \( B \) is:
\[
B = \frac{9.3}{3 \times 10^8} = 3.1 \times 10^{-8} \, \text{T}.
\]