I have to create new language with languages $L_a = \{a\}, L_b = \{b\}$ and with set operations which contains only wordswith even lengthwith odd length and first letter not equal to last letterSo my solution for part 1 is this. Am I rigth?$L_n = \{aa, bb, ab, ba\}* = (L_a.L_a \cup L_b.L_b \cup L_a.L_b \cup L_b.L_a)*$Regarding to part 2, I dont have any simple solution. In this language must be words like these: $abb,bba,baaaa, ababb$ etc. So before first and last word must be only word with odd length. But I dont know how to write it. 
How to create new language with languages $L_a = \{a\}, L_b = \{b\}$ and set operations?
formal languages
Part 1. is ok. The idea behind is that you have to take the letters 2 by 2 and then make the star closure. Call $L_e$ the language of even words you just defined.Part 2. Assume for a moment to have defined $L_o$ the language of words of even length from $L_a$ and $L_b$. Then to have even length and different initial and final letter you can just define$L=L_a\cdot L_o\cdot L_b\cup L_b\cdot L_o\cdot L_a$Remark that words in $L$ have length at least 3 since $a$ starts and finish with the same letter...To define $L_o$ the idea is that an even word is just to exploit $L_e$:$L_o=L_a\cdot L_e\cup L_b\cdot L_e$.