You can use that formula, but you need to do a little work to make it meaningful. What are x,y, and s? Draw a picture. Think about what those distances actually mean. You may benefit from drawing multiple diagrams. Draw the initial positions of the two ships. Draw the positions of the two ships after 90 minutes. You may even want to draw a diagram of the positions of the two ships at some point in between. If you are using , then you want x, y, and s to be the sides of a triangle. In the "initial" diagram, this triangle will be difficult to see because one side will have length zero.