The complete information for the question was not provided by the student, however we can assume that the box is either to the left or to the right of the equal sign:
If it's to the left, we have:
Box + 5 = 9
Box = 9 - 5
Box = 4
If it's to the right, we have:
5 = 9 - Box
5 - 9 = - Box
-4 = - Box
4 = Box
Answer: No matter if the box is to the left or to the right of the equal sign, the number in the box that makes the equation true is 4