The question ask to determine these in 3 different mediums: a vacuum and two different plasma's. The full question is attached.© BrainMass Inc. brainmass.com November 30, 2021, 2:42 am ad1c9bdddf
The answer is given clearly in the word file attached below.
(i) The phase speed of wave in vacuum is the velocity of light. That is 3*108 m/s
Wavelength = velocity of light / frequency
f= 2.45GHz =2.45*109 Hz
 = (3*108)/(2.45*109)
= 12.24 cm
(ii) Given data Number density (n0) = 7*1016 /m3
Wave frequency (f) = 2.45GHz
Phase speed of a wave in a plasma is given by Vp= c/SQRT(1- (fp/f)2)
Where plasma frequency fp = 9*SQRT(n0) Hz , n0 in number of electrons/ m3
A clear cut solution is attached with step by step explanation.