Suppose a car is traveling in the negative x-direction and comes to a stop. What is the sign of that car's acceleration? How do you know?

We have equation of motion v = u + at

Where v = final velocity, u = initial velocity, a = acceleration, and t = time

In this case car is travelling in - x direction

Velocity of car = displacement / time

Since displacement value is increasing in negative x axis it's initial velocity is negative

And it's final velocity is zero since it comes to rest, and time is also positive

So, v = u + at = > 0 = - u + at

So, a = u/t

Which is positive and along positive X - direction