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