So in part a we need to find the altitude of the hill. We know its difference with checkpoint 2 is 338 feet. This means that:
If I use h for the altitude of the top of the hill and -193ft for the altitude of checkpoint 2 as the table states we get:
Then the altitude of the top of the hill is 145ft.
In part b we need to find how much lower is checkpoint 2 than checkpoint 4. This basically means that we must find the difference between their altitudes. This is given by taking checkpoint 4's altitude and substracting checkpoint 2's altitude:
Then checkpoint 2 is 2668ft lower than checkpoint 4.