Can someone help me on a few questions which I got stuck on and show me how you got the answer?

1. Evaluate: Sigma sign n on the top, 1 underneath then in the middle 2r-7.

2. The first term of an aritmetic series is 14. The common difference and the sum of the first n terms are both -6. Find n.

3. The first three terms of an arithmetic series are 2x+1, 3x+2 and 5x-1. Find x.

Thanks in advance