-How can investigators use quadratic modeling to determine a car's speed?
-What is a car's stopping distance (Reaction distance+Breaking distance) for a car going 70 mph? 100 mph?
-Find the speed of a car whose stopping distance is about 250 feet. 135 feet.

I need help with these... thanks!

Obvoiusly there is more information you have been given for these problems than you've lised here. However, the basic concept is that the distance covered by the when it brakes has two components:

1. Reaction distance - which depends on how long it takes for you to start applying the brakes. Call the time it takes you to react R, then the distance covered during reaction is R * v

2. Braking distance - once the brakes are applied the distance the car travels before coming to a stop is proportional to the square of the initial speed. In other words, distance during braking is B*v^2, where B is some constant.

So the total distance the car travels is:

D = R*v + B*v^2

Perhaps elsewhere they have given you further data, such as the reaction time or th total distance it takes to brake from say 30 MPH. From data like that you can solve for R and B, and then answer the remaining questions.