The drilling of each well has an associated cost, and the object is to determine the underlying shale surface for the least cost. It is the orientation and direction of tilt of this straight line that should be reported. This captured image should be incorporated in your writeup of this project.
Cost is important, but getting the right answer is more important.
Make sure you've sunk enough wells to be pretty confident about your conclusions. It's OK to work just with the flat topography. Here you'll have to subtract out the topographical features to deduce the underlying stratum, because the well depths are listed as the distance from the surface to the underlying stratum.