\[S_{по\ теч} = 18\ км;\ \ \ \ \]
\[S_{прот.\ теч.} = 15\ км;\ \ \ \ \]
\[V_{соб}. = 2\ \frac{км}{ч}\]
\[t = \frac{S}{V}\]
\[\frac{18}{x} = \frac{15}{x - 2}\]
\[18 \cdot (x - 2) = 15x\]
\[18x - 36 = 15x\]
\[3x = 36\]
\[x = 12\ \left( \frac{км}{ч} \right) - собственная\ \]
\[скорость.\]
\[Ответ:\ 12\ \ \frac{км}{ч}.\]