Answer:
you're given a *rate* of decrease and then the *extent* of the "cause". SO the rate is 10C/1000m (right? You understand why this is a rate? You go 1000m up, the temperature drops 10C. And it happens continually, bit by bit, as you climb. That's a rate!
But, you don't stop with climbing just 1000m: you climb 1000m twice *more*. So, how much has the temperature dropped in all, by the end?
You really should be able to put this together as 10C/1000m x 3000m = result. It will be in units of (degrees) C.
Now, finish the problem and get down the mountain, before you freeze (also, winds kick up more on mountains -- a double whammy).
Explanation: