It should be noted that motion simply means the change in the position of an object for a period of time.
Your information is incomplete as the graph isn't given. Therefore, an overview of motion will be given. Motion can be described in terms of velocity, displacement, distance, etc.
The acceleration of a car is the rate at which the velocity changes. In such a case, even though the speed is constant, an object can still accelerate.
Also, a constant speed of a car means that the speed of the car is the same as it neither increases nor decreases.
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