Master method recurrence question
I think the question is asking how to translate $f(n)$ into Big-Theta notation to be used for the Master Theorem. You have the right idea about switching between notations. 

Another way to write the Master Theorem is as follows: 

$T(n) = aT(n/b) + \Theta(n^d)$

So, yes, if you have a linearly growing function, then $f(n)$ may be substituted with $\Theta(n)$.

Then you may proceed with your comparisons.