Part b can be calculated in a simpler way. Since it is the wind which blows the plane off course, we can simply calculate the distance travelled by wind in 10 minutes:
100 km/h * 10/60=16.7 km.

Hi mjftian, yes, you're quite right, and thank you for the comment. In the vector triangle for part b), the amount by which the plane is blown off course could be labelled better as $d_{AG}$ for displacement of the air with respect to the ground. Thinking of it that way, its magnitude will clearly be the air speed multiplied by time, as you mentioned. Ah well, hopefully there's value for students to see the cosine law in action, since that's sometimes a necessary technique, and an important part of the problem solving toolbox.

All the best,
Mr. Dychko