however i have no idea where the 300,000 km comes from....
The formula you are using is lambda = v/f
lambda = wavelength (m)
v = speed of signal (m/s)
f = frequency (Hz)
In this case, radio waves travel at the speed of light, 3x10^8 metres per second or 300,000 km/s.
That's where 300,000 comes from.