Why do this problem?
is tricky to do but can form an interesting challenge to those who need it. It requires much addition & subtraction as well as the making and testing hypotheses.
Have you tried adding the diagonals?
Can you get a clue from the way numbers are selected if you look carefully at the way this is forced to happen?
Does it help to look at the diagram in the hint section?
Learners could make their own puzzle matrices.
Suggest doing a easier problem such as Shapes on the playground