Question

A speed-time graph shows that a car moves at 20 m/s for 15 s. The car's speed then steadily decreases until it comes to a stop at 40 s. Which of the following describes the slope of the speed-time graph from 15 s to 40 s?

Answer 1

There is nothing "following".

Any portion of a graph that represents decreasing speed
has a negative slope. That is, as you travel along the line
from left to right, the line sinks lower on the graph.