how can we calculate wavelength and frequency of a standing wave of different amplitudesfor ex:kcos(ax+bt)-0.8kcos(ax-bt)

In the standard eqn, omega denotes angular frequency.And frequency can be found from angular freq.Freq.=Ang. freq./2pie

In your expression anular freq. of the two waves are the same and thats the reason they are able to form a standing wave.

Once you have the frequency, you can easily find wavlength as wavelength=wave velocity/freq.