Three young children are fighting over where to take
 Super Dog. Child X pulls the wagon, trying to go in the
 direction of his house. Child Y pushes on the wagon
 towards X's house. Child Z pushes the wagon in the
 opposite direction. The forces the 3 children exert on the
 wagon are 300 N, 500N, and 200N (not in any order).
 The wagon isn't moving, how much force is child Z
 pushing with?