On a​ planet, a rock is dropped from the top of a​ 400-foot cliff. After 1​ second, the rock is traveling feet per second. After ​seconds, the rock is traveling feet per second.
a. Assume that the relationship between​ time, t, and​ speed, s, is linear and write an equation describing this relationship. Use ordered pairs of the form​ (time, speed).
b. Use this equation to determine the speed of the rock seconds after it was dropped.