The question ask to determine these in 3 different mediums: a vacuum and two different plasma's. The full question is attached.
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.