Knowing that the final score was 7-4 means that at inning 5 the winning team had somewhere between 0 and 7 runs and the losing team had somewhere between 0 and 4. That's 8 possibilities for the winning team and 5 for the losing team, or 8 x 5 = 40 total possibilities.

As for the second question - what you wrote is the calculation for NC2, but I'm not sure what you meant by "write an expression for N (nC2)."