I am absolutely dsperate for help. I've read the sections in the book, can work the practice problems and feel like I understand it until I go to do my homework, then I get the problems wrong almost every time. This has been going on for awhile now but I've just absolutely had it; I'm going crazy.

Right now, I've been working this problem:

A government agency estimated that air bags have saved over 14,000 lives as of April 2004 in the United States. (They also stated that air bags have been confirmed as killing 242 people, and they stress that seat belts are estimated to save 11,000 lives a year.) Assume that a car crashes and has come to a stop when the air bag inflates, causing a 75.0 kg person moving forward at 15.0 m/s to stop moving in 0.0250 seconds. (a) What is the magnitude of the person's impulse? (b) What is the magnitude of the average force the airbag exerts on the person?

Should I not be able to find a. by multiplying mass times velocity to get force then multiplying that by the elapsed time? And wouldn't the answer then be 28.125? Also, for b., wouldn't it just be the opposite of the force being put on the airbag by the person? In other words, wouldn't it be -1125 since 75*15 is positive 1125?

I am literally in tears over this because I have HAD it with getting the textbook practice problems right and then failing this stupid online homework every time without having any way to ever see what the correct answers were or if I'm even going about the problems the right way. Please, please someone help me...
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