Objects that move with a constant velocity (same speed ... same direction) experience a balance of forces. Oppositely-directed forces are of the same strength. On the other hand, objects that accelerate experience an unbalanced force that is directed in the direction of the object's acceleration.
 

You are provided with an acceleration value for an object. You are then told that the net force is changed to twice its original value ("doubled"). You are asked to determine the new acceleration that results from this "doubling". Newton's second law states that the acceleration is directly proportional to this net force, This means that an alteration in net force would alter the acceleration value by the same factor. Thus, a doubling of net force causes a doubling of acceleration. And so your task is to take the given acceleration value and change it by this doubling factor.