Would you please explain what does $n_2$ represent? I only know that n_1 is the problem size on a single processor. So n_2 is the problem size on 2 processors?
You can apply the same speedup and efficiency definitions starting from an n_2
(or other size).
Still a little confused about the problem, can I understand the n2 part as calculating speedup according to the formulae
while replacing all t1(n1) with t2(n2)?
Sorry, but this goes a little too far in the discussion. I think that you need to apply some judgement how to adapt the formula. It is part of the question.
Please do not go into such details when you post questions on assignments.