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 .