If the Nyquist rate for xa (t) is Ωs, what is the Nyquist rate for xa(2t)? Will it be the same, increase 2 folds or decrease 2 folds?
I'm assuming that this is homework, so I'm not going to give you the answer, but will try to help you get there yourself.
With $xa(2t)$ everything happens twice as quickly (what was time $1$ with the original signal is time $2$ with the new signal, time $2$ becomes time $4$, etc.). Given that everything is happening twice as fast as before, what would that do to the Nyquist frequency?