|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