Addison,
Imagine that you are on the bike. You ride in one direction 4.5 miles. Now, you are 4.5 miles from where you started at the bottom of the hill. You turn around and ride back the way you came. You are riding down the hill. As you ride, you are getting closer to where you started. You ride until you have gone 2.7 miles back down the hill. So, your position is
4.5 miles - 2.7 miles = 1.8 miles (from the bottom of the hill)
That is, you are 1.8 miles from where you started at the beginning of the problem.
I hope this helps you understand the question. I find it is often much easier to understand problems like this if I imagine myself doing what the problem describes.
