The net force experienced by an object is the collective effect of all the individual forces. Since force is a vector, direction must be considered. A leftward force subtracts from the effect of a larger rightward force and a rightward force subtracts from the effect of a larger leftward force. Forces directed in the same direction add together to produce the combined effect of a larger net force.
 

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