Information that we have:
The temperature at the base is 28°F
The temperature decreses 2.5°F for every thousand feet of elevation.
the temperature at the summit is 24°
The equation to solve this problem will be, the initial temperature at the base, which is 28, to that we need to substract the temperature for each thousand feet (the distance above the summit will be "x") and that must be equal to 24.
This is because initially we have 28° and we substract 2.5 for every 1000 feet.