Find the wavelength for a typical AM radio with a frequency of 900 kHz?
Find the wavelength for a typical FM radio with a frequency of 99.6 MHz
I'll use the value of the light speed in vacuum which I have at hand (this value is from 1972 so it may differ a little bit today): $\displaystyle c = 299,792,456\ \frac ms$
Let f denote the frequency and l the wavelength then you know that
$\displaystyle f \cdot l = c$
Plug in the values you already know:
$\displaystyle 900 \cdot 10^3\ \frac1s \cdot l = 299,792,456\ \frac ms$
$\displaystyle 99.6 \cdot 10^6\ \frac1s \cdot l = 299,792,456\ \frac ms$ . Solve for l.
(For confrmation only: 333.103 m; 3.01 m)