Vectors quantity have magnitude and direction, which means that they can be solved arithmetically with their corresponding signs. In this case, we can simply add the two velocities to get for the final velocity of the plane. But first let us determine the signs of each velocity value.
As a reference, we take the direction of the plane to be going the positive y-axis. Therefore it is going up and positive. Now take note that the wind is a “tailwind” which means that the wind is going WITH the direction of the plane, therefore it is also a positive. Now knowing that, we can add the two:
Final velocity = 300 km / hr + 20 km / hr
Final velocity = 320 km / hr