If the problem says "space", that usually means that they are talking about volume. The maximum length the box can be is 8.4 inches. To find volume, you do length * width * height. You know the length and height, as well as the volume. You can replace the width with "x". Your equation should be:
11.5 * 8.4 * x = 272.8
Then you can just solve for x by dividing both sides by (11.5 * 8.4). The answer should be about 2.82 inches.