it comes from http://www.foundalis.com/res/bps/foundal/p162.htm
What you need to do is to distinguish the patterns on both sides, what is the common feature on the left and what is the common feature on the right
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Not sure about the first one, but for the second:
If you count along each grid row by row, then the first group has black squares evenly spaced (5, 6, 4, 7, 2, 9), whereas the second group does not have such even spacing.
Edit: after a bit of googling, it appears that the first one has a couple of possible solutions (taken from a reddit thread):