Answer:
At its uppest and lowest position
Step-by-step explanation:
In a simple harmonic motion, the acceleration is given by:
![a(t)=-\omega^2 x(t)](https://img.qammunity.org/2020/formulas/physics/high-school/9euo5ipl80xku03vgksito5ihqaex9lezt.png)
where
is the angular frequency of the motion
x(t) is the displacement of the object from the equilibrium position
So we see that the acceleration is directly proportional to the displacement. Therefore, in this situation, the acceleration of the car will be greatest when the displacement is also greatest: so, when the car is at its uppest position and when it is at its lowest position.