Hope this helps: you know O is in the last row since P is beside R and an X, it has to be XPR or RPX... and that means, it can also NOT be in the last row, as O is already there. Then you read that R is in a row behind N, so since R cannot be in the 3rd row, the XPR/RPX has to be in the second row, ...

I'm a bit confused. I thought for contropositives you were supposed to negate and reverse? But you didn't negate the contropositive you did...

Explain?