Ecuation

[simple root]

For this question, you would want to use the formula
average speed km/h = distance km ÷ time h
Let's call the distance D, and the time on the way up T
We can form the first equation for the way up
3 km/h = D km ÷ T h
Then we can form the second equation on the way down [I'm assuming "twice as high" means it takes him longer, so 3*2 -- I assume this is a language issue, but "twice as high" is unclear to me. I might assume incorrectly, and it's actually 1.5km/h down]
6 km/h = D km ÷ T+1 h
This is not the problem solved, but it provides a starting point. Does this help enough for you to finish it on your own?

@sudden carbon

[strange salmon]

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