1. To find for the value of the slope, we need to have the graph. But you forgot to include the graph in your question. So I will just help you find for it.
In the graph, find for any two pairs of x and y. Then use the equation:
m = (y2 – y1) / (x2 – x1)
where m is the slope, y2 is the 2nd y, y1 is the 1st y, x2 is the x of the 2nd y, x1 is the x of the 1st y
2. Actually the slope m represents the:
the cost per mile traveled