The formula for Marginal Revenue is:
As you can see, you are only dealing with 1 hour of work, so we can plug in what we know:
Now we must ponder what happens to her pay as the amount of hours she works changes. Does it go up? Does it go down? or does it stay the same?
It's obvious to see that no matter how many hours she works, her pay will still be 120 dollars per hour. So, therefore:
There you go.