7.21 ft/s^2
Since you're looking for average acceleration, you can simply divide the change in velocity by the time. To make the calculation more reasonable, first convert the speed of 173 mi/h into ft/sec by multiplying by 5280 to convert from mi/h to ft/h and then dividing by 3600 to convert from ft/h to ft/s.
173 * 5280 / 3600 = 253.7333 ft/s
Now divide the change in velocity by the time in seconds.
253.7333 ft/s / 35.2 s = 7.208333 ft/s^2
Rounding result to 3 significant figures gives 7.21 ft/s^2
