this is the most basic problem in relativity... are you having a hard time understanding relativity or you just can't figure out how to do this problem...
relativity usually puts in a advance physics class. since you are learning relativity now, i believe you have already suffer through the classical mechanic and electrostatic and thermal physics those basic stuff... and you have already develope a skill to deal with physics problems. This problem is not tricky at all. plug the number in appropiate formulas and the answer will shown up instantly... if you really can't do this problem, at least show me some of your work or let me know which part you stuck in...