Transylvania is in Europe and is a part of Romania, and which borders Hungary, the Czech and Slovak Republics. That part is simple. There are three points to remember for the question where is Transylvania?
-- Transylvania is in Europe.
-- It is a province in Romania.
-- It is a apart of "eastern" Europe.
More to the point when people ask where is Transylvania they might as well ask the next question of why do you know about Transylvania:
Transylvania is a part of my history and so are the legends and people in that land. I find that when I mention that yes, Transylvania has water buffalo, people will ask: where in Transylvania can you find water buffalo ( central Transylvania, and also a bit in northern Transylvania. the land is lush so they can eat their fill).
Still it is interesting to learn about new people and places and sometimes, Dracula does come into the picture but then where in Transylvania can you find Dracula? ( In terms of popular culture and Transylvania....)