To find your constant of variation, you just need to figure out what you multiply the x by in order to get f(x). In other words, what do you multiply the 3 by to get 6? What do you multiply the 7 by to get 14? Another way to think of it is to divide f(x) by the x to get your constant.
Her elevation went down too quickly, if she is going down 300 ft per hour at a constant rate, at 1 hour, she should be at 2700 feet. It should take her about 10 hours to get down the mountain.