I think the "2n terms" should have been "3n terms", because with 
2n terms, there will have to be three formulas, depending on whether 
2n is a multiple of 3, 1 less than a multiple of 3, or 1 more than 
a multiple of 3.

So I'm going to do the problem assuming that the 2n should have been 3n,

OK?  Let me know.

It's the sum of two arithmetic sequences:

S1 = 2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+...+3n+1 (to 3n terms) 
S2 =    -4    -7    -10      -13      -16+...      (to n terms   

There are 3n terms of the first sequence and n terms of the second sequence.

We use the formula:



We substitute  for , , 




 
 = the sum of the 1st sequence.

We use the formula 



to find the sum of the 2nd sequence

,  







So the required sum is







Sum of the first 3n terms = 

Edwin