If we call y the height of the plane, we know when the time (x) is 0, the height is 30000. So let's write that in:
y = 30000 + (0)
Then, for every increase of a minute (x increases by 1), the height reduces by 2000. This can be written as:
y = 30000 - 2000x
Using what we know about y = mx+c equations, the slope of the line is -2000, and the y-intercept is (0, 30000).