No problem, it happens all the time. Ok, the first thing you should do is get a LCD in the top and bottom of the larger fraction:
The LCD in both the top and bottom will be (x+3). Think of the non-fraction numbers as , this will help you see how to evaluate their LCD
So you will have a fraction over a fraction, with LCD both (x+3). Then, just multiply the top by the reciprocal of the bottom to get rid of the big fraction. This will leave you with a new fraction, which you can multiply out giving you your final LCD
Good luck!
To learn the methodology, try studying one or two online lessons covering how to simplify complex fractions. Make sure you can follow the worked examples.