Answer
The parallelogram rule is discussed on pages 28-29, and relative velocity is discussed on page 51.
The rain's velocity relative to your face equals the vector sum of its velocity relative to the ground, and the ground's velocity relative to your face. These latter two velocities are at right angles.
The resultant of the perpendicular 3 m/s and 4 m/s vectors is 5 m/s, by the Pythagorean theorem.
$$(3 m/s)^{2}+(4 m/s)^{2}=v^{2}$$
$$v=5 m/s$$
