Q:

How to combine two list with the different element?

How to combine two list with the different element? The problem like this:

A:

You're looking for the (\\) function:
[2,4,6,8,10] \\ [3,4,5,6] == [2,8,10]

Haskell comes with a lot of built-in functions. Especially for simple tasks like this, you'll often find one that does the job. But how do you find one?
The first step is to determine the type of your desired function. In this case you're looking for a function that takes two lists with elements of the same type (a) and produces a list with elements of that same type:
[a] -> [a] -> [a]

Next search Hoogle for functions that match that type. (\\) is near the top of the results. It includes the following documentation:

The \\ function is list difference (non-associative). In the result of xs \\ ys, the first occurrence of each element of ys in turn (if any) has been removed from xs. Thus
(xs ++ ys) \\ xs == ys.

It is a special case of deleteFirstsBy, which allows the programmer to supply their own equality test.

