It's very simple. With A as the frame of reference, this picture is the S/T diagram of B relative to A (distance/time diagram).

Then the slope of two straight lines is the velocity of b relative to a.

At first, A did not move, and 1 hour A started, so 1 hour, the slope did not change, and the slope did not change later.

There is a sudden change in the slope at 1 hour, that is, the point A is 1 hour.

The speed of B in the previous hour was 40km, which means that B was 40 km/h..

The velocity of a relative to b is 200/( 12- 1), and the absolute velocity of a is 40+200/11= 640/1/km/h.