Chapter 1, Equations and Graphs - Section 1.4 - Solving Quadratic Equations - 1.4 Exercises - Page 124: 82

$9mph$

If the speed of the southbound boat is $v$, then the equation of their distance by the Pythagorean Theorem: $(2v)^2+((v+3)2)^2=30^2\\4v^2+(2v+6)^2=900\\4v^2+4v^2+24v+36=900\\v^2+v^2+6v+9=225\\2v^2+6v-216=0\\v^2+3v-108=0\\(v+12)(v-9)=0$ Thus $v=-12$ (negative thus not possible) or $v=9$.