To calculate the net force acting on the plane due to the tailwind, you would add the two forces together.
Therefore, the correct response is:
Add the two forces together.
This is because both the plane's speed and the tailwind are in the same direction, which means they combine to give the total effective speed of the plane. However, it's important to clarify that net force is normally calculated based on mass and acceleration, but in this context, where we're determining speed, "adding" is the appropriate action.