As shown in the figure, the x axis represents the hour (h) and the y axis represents the distance (km).

Then understand the function image according to the meaning of the topic.

As shown in the figure, the train has been running for 4 hours (at a constant speed) from point A to point B for 900km. When you arrive at point b, it is 0km away from point B.

The train goes back from point B (at a constant speed), and when it reaches point C, the speed of the train changes, which is slower than before.

Finally, you can solve the corresponding functional relationship (because LZ you didn't give a complete topic, the above is purely my own understanding. )

Excuse me, this topic seems to be in the first volume of the eighth grade mathematics "experimental class"