Question

As she climbs a hill a cyclist slows down from 25 mi/hr to 6mi/hr in 10 seconds what is her acceleration

Answer 1

|Acceleration| = (change in speed) / (time for the change).

Change in speed = (6 mi/hr - 25 mi/hr) = -19 mi/hr
Time for the change = 10 sec

|Acceleration| = (-19 mi/hr) / (10 sec) = -1.9 mile per hour per second

Admittedly, that's a rather weird unit.
Other units, perhaps more comfortable ones, are:

                 -6,840 mi/hr²

                   -2.79 feet/sec²