The median is the middle number. To have the median fall in the category '2', the number of students on the left and right of the median student must be equal.
On the left of '2', we have the students who answered '0' and '1' (so we add them up 2+8)
BUT: maybe some of the students who answered '2' also fall on the left of the median student.

To get the largest value of x (which is on the right of the median), you must try to get the left side of the median to have as many students as possible, without crossing '2'. So we further add all the students who answered '2', except for one (we need to leave one because the median is '2').