Answer
The speed over the ground is 141 km/h.
Work Step by Step
This is discussed on page 51. See Figure 3.12, and use the Pythagorean Theorem to find the length of the resultant vector.
$$(100 \,km/h)^{2}+(100 \,km/h)^{2}=x^{2}$$
$$x = 141 \,km/h$$
As a side note, the plane moves at an angle of 45° from its original direction, due to the stiff crosswind.
