The given information is:
Initial height of the plane above the ground= 30,000 ft
Descending rate= 2000 ft/min
To identify the variables in this situation let's state that X is the independent variable and Y the dependent variable.
The time doesn't depend to the height of the plane, but the height of the plane actually does depend on the time.
Then, the independent variable is time (minutes) and the dependent variable is the height of the plane.
X=time (minutes)
Y=height of the plane above the ground(ft)
The related equation is:
As you have a descending rate, it means the height is decreasing, then 2000 has a negative sign.