The linear function that models the problem is
y = 250x + 30,000
How to model the required equation
The linear equation is written in the slope intercept form
y = mx + c
definition of variable to suit the problem
y = output variable (height at any given time)
m = slope
x = input variable (time)
c = y intercept
The equation as described in the problem is represented using linear function as as follows
c = 30,000 feet
m = 250 feet per minute
y = 250x + 30,000