(1) If you do not provide full code to replicate the example then it is impossible to really say much, short of retyping from an image (which I for one will not do).

(2) You will have a better chance if you make the upper bound an explicit number, 10, in this case, rather than encode it in the equations. Solve sees the equations but Sum does not, and Solve needs Sum to fully evaluate (to something explicit rather than a Sum).

(3) It is not a good idea to use N as a variable name, since it is also a heavily used function in Mathematica.