What do you mean by 2n^-n with only a caret ^ to indicate 
an exponent but then there is no exponent after it?

Maybe you meant  and should have written 2n^2-n. 
Is that it?  I'll guess that's what you meant.

The first term has to be just the "sum of the first term
only", [even though nothing has been summed!] but that's
how we consider "summing" when there is only one number.
So the first term has to be 2(1)^2-1 or 2(1)-1 = 2-1 = 1.
a1 = 1.  Then for n > 1



The sum of the first n terms = 
The sum of the first n-1 terms = 

So, nth term =  
               
                 

               

               

               

               

Edwin