Final answer:
The slope of the blue curve at point b represents the instantaneous velocity of the plane at that point, which is measured in feet per minute.
Step-by-step explanation:
The slope of the blue curve at point b represents the instantaneous velocity of the plane at that point. To find the slope, we draw a tangent line to the curve at point b and calculate the slope of this line. The slope of the tangent line represents the rate of change of the position of the plane with respect to time, which is the velocity. Since the blue curve represents the position of the plane over time, the units of the slope are feet per minute, indicating the rate at which the plane is moving.