To calculate the plane's net force with the given conditions, you should add the two forces together.
The speed of the plane (70 km/h) and the speed of the tailwind (5 km/h) both act in the same direction, so the net speed of the plane is the sum of the two speeds:
70 km/h (plane) + 5 km/h (tailwind) = 75 km/h (net speed).
So the correct response is: Add the two forces together.