Hey Mariah. So, first start the problem by looking for the electron configuration of La as an element, without the 3+ charge. Once you look for it, you should obtain [Xe] 6s2 5d1 or [Xe] 5d1 6s2. Now that you have this configuration for La, you must find it for the ion La 3+. So, you are making it more positive and less negative, so we must remove electrons. Take away 3 electrons from the outermost shell. So you will have to take 2 electrons from the 6s shell and then one more from the 5d shell.
This means that the electron configuration for La 3+ should be the same as the one for Xe. So it should be [Xe]. Hope this helps!