TeX source:

\begin{aligned}\mathrm{T}=\text { total time } &=(\text { rowing time from boat to } \mathrm{P})+(\text { walking time from } \mathrm{P} \text { to } \mathrm{B}) \\&=(\text { distance from boat to } \mathrm{P}) /(\text { rate from boat to } \mathrm{P})+(\text { distance from } \mathrm{P} \text { to } \mathrm{B}) /(\text { rate from } \mathrm{P} \text { to } \mathrm{B}) \\&=\sqrt{\mathrm{x}^{2}+4^{2}} / 3+(2-\mathrm{x}) / 5=\frac{\sqrt{\mathrm{x}^{2}+16}}{3}+\frac{2-\mathrm{x}}{5}\end{aligned}