Thank you for the opportunity to help you with your question! the solution is as such:

The rate of a change is always the slope of the curve and here we can see that the slope of the line joining (1,0) and (0,1) is slope = (y2 - y1)/ (x2 - x1)

So slope is -1.

So its true that the rate of change is - 1

Please let me know if you need any clarification. I'm always happy to answer your questions.awaiting a positive feedback. thank you.