What is the approximate distance between portland, oregon
(45° north, 122° west), and san francisco, california (37° north, 122° west). use 3,960 miles for the radius of earth.

5669.71 km

Working
The two cities are located on the same longitude. 
The formula for the distance then is;
Distance = *2*π*3960
where ∅ is the latitude difference-add the latitudes as they are on different sides.
Distance = *2**3960
                =5669.71 km

Were is the diagram?