Belfast city to londonderry?

i just receive a letter for a placement interview with Invista. The letter did not mention where the interview be set, but supposed at their company in londonderry.Since my car had been tow away by mr. policemen because i drive recklessly and with no insurance last summer, i wonder what other best transportation to get there? and i dont know much about londonderry, is it easy to get to places there? i mean is cab available everywhere tehre?