What is meant by L1L2 ? Does the n have to be the same for both? So, aabbcc is an element of L1L2 and aabbcccc is not? How about the first problem - Epsilon. Is it an element of L1L2? Since n>0 in L1 and L2, Epsilon should not be element of L1L2... But what is meant by L1L2? That's the key to solve all four problems...
Hi.
Nop, it does not have to be the same $n$. $L_1L_2=\{xy: x\in L_1,y\in L_2\}$. So, if you want to see if a string is in that set, you want to factor it in two parts, the prefix part must to be in $L_1$ and the suffix part must to be in $L_2$. As an example, $aabbcccc\in L_1L_2$ but $aabb\not \in L_1L_2$.
Hope this helps.