The correct answer is the first option.
If you want to use elimination, you can sum the two equations for example, so that the x's simplify:



Plug this value for y in one of the equations to derive the value of x:

So, the solution is 
You are adding by 5 in the sequence.
If f(1) is -7
f(2) = -2
f(3)= 3
f(4)=8
f(5)=13
f(6)=18
Therefore, f(7)=23
90 games since each team would be playing 9 games (one against every other team)