A rightward-moving car accelerates from 55 mi/hr to 25 mi/hr in 4.2 seconds. Determine the magnitude of the average acceleration of the car in mi/hr². Enter a numerical answer with three significant digits.
a. -11.9 mi/hr²
b. -8.10 mi/hr²
c. -6.19 mi/hr²
d. -4.76 mi/hr²