When x is from 0 to 4, y and x are in direct proportion, that is, point p moves from b to b, that is, BC=4.

X from 4 to 9 means that P moves from C to D, so CD=9-4=5.

AB=CD=5 in rectangular ABCD.

So the area of triangle ABC is S=AB×BC÷2=5×4÷2= 10.

Is there anything else I don't quite understand?

You can ask