Answer
The parallelogram rule is discussed on pages 28-29, and relative velocity is discussed on page 51.
The plane's velocity relative to the ground equals the vector sum of its velocity relative to the air, and the air's velocity relative to the ground.
The two velocities are perpendicular. The resultant of the perpendicular 120 km/h and 90 km/h vectors is 150 km/h, by the Pythagorean theorem.
$$(120 km/h)^{2}+(90 km/h)^{2}=v^{2}$$
$$v=150 km/h$$
