How do I find the average rate of change of two points in a contour map?

The distance between $A$ and $C$ is to be calculated by the Pythagoras theorem. The changein the value of the fiwld is to be read from the diagram to be $phi_C - phi_A =-2a$. The average rate of change is then $$ Delta=frac{2a}{sqrt{4^2+1}} = afrac{2}{sqrt{17}} $$ $$ a = -14 implies Delta = -14frac{2}{sqrt{17}} = -frac{28}{sqrt{17}} $$