Answer
$3\frac{ 1}{3}\;m$.
Work Step by Step
Frequency $f=9\times 10^7$ waves per second.
Speed $s=3\times 10^8$ meters per second.
Wavelength $\lambda=\frac{s}{f}$.
Substitute both values.
$\lambda=\frac{3\times 10^8\;m/sec}{9\times 10^7\;/sec}$
Simplify.
$\lambda=\frac{3\times 10^8\;m}{3^2\times 10^7}$
$\lambda=\frac{ 10^{8-7}\;m}{3^{2-1}}$
$\lambda=\frac{ 10\;m}{3}$
$\lambda=3\frac{ 1}{3}\;m$.
