Instead of using x and y for our linear equation, let's use (t, h) for time and altitude. When the wheels are lowered, the time is t =0 and the altitude is h = 1500 feet. That gives us the point (0, 1500). Four minutes later, the plan is on the ground, so t = 4 and h = 0, giving us the point (4, 0). Use these two points to write a linear equation.
Slope is
The "y-intercept" is 1500 (it's really the h intercept since we renamed y to be h).
Equation:
Now, the meaning of the slope -375 is that the plane is losing 375 feet in altitude per minute. That's because in the slope calculation, you divided a difference in FEET by a difference in MINUTES.