First we need to count the total number scores. This can be done from the stem and leaf plot. The total number of scores are 19. The total number of values is odd, so the median position will be:

Thus the 10th score is the median score for the class of Mr. Robert. The 10th score from the stem and leaf plot is 81.
Thus 81 is the median score of Mr. Robert's Class.
The opposite of 4 is negative 4, or -4
Ben is taller
Ben: 5 5/16 feet = 1.61925 meters
Marcus: 5 7/24 feet = 1.6129 meters