book Maths Puzzle Numerical Ability

No matter how I try, I am not understanding this type of problem....
3x -2y + 5z
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2y -3x -5z

This is how I try....
3x (3x-2y + 5z) -(2y + 5z)
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while covering a diatance of 24 km, a man was noticed that after walking 1h and 40 minutes, the distance covered by him was 5/7 of the remaining distance. What was his speed in meters per seconds.