Question

use newton's first law of motion to explain how wearing a seat belt in a moving car could help prevent an injury.

Answer 1

Well while car is moving with some velocity, we also move with same velocity.
Now according to newton law of inertia if car is stopped using brakes our body will continue to move with same velocity and thus we will hit the windscreen of car, if we have put on seatbelt then our body will not move forward rather it will stop (much like the car is stopped when brakes are applied)
Hope it helps