revise the chapter of the arithmetic sequences and find the formula that gives the sum of n terms of an arithmetic sequence
apply this formula twice ..
once for 2n terms and second time for n terms. then subtract the two expressions and your answer will come very easily.
dont forget that the 2nth term of the arithmetic sequence is a-(2n-1)4 while the nth term is a+(n-1)4 . a represents the first term of the sequence but it will cancel with the subtraction.
I did it my self and it is easy...try it.