You throw a ball straight up at 45 m/s. how many seconds elapse before it is traveling downward at 9 m/s?

In order to change from 45 m/s up to 9 m/s down, the velocity

of the ball has to change by (45+9) = 54 m/s downward.

Gravity adds 9.8 m/s of downward speed to any falling object

every second.

So the change takes (54/9.8) = 5.5 seconds.