Answer
(a) 2000 s, (b) 1800 m
Work Step by Step
Please see the attached image first.
Let's take,
Swimmer's velocity with respect to water = $V_{SW}$
Water velocity with respect to ground = $V_{WG}$
Swimmer's velocity with respect to ground = $V_{SG}$
(a) Let's apply the equation $S=ut$ to the swimmer with respect to water to find the time to cross the river.
$S_{SW}=u_{SW}t$ ; Let's plug known values into this equation.
$2800\space m=1.4\space m/s\times t$
$t=2000\space s$
(b) (a) Let's apply the equation $S=ut$ to the water with respect to the ground to find the distance x that the swimmer is carried downstream while crossing the river.
$S_{WG}=u_{WG}t$ ; Let's plug known values into this equation.
$x=0.91\space m/s\times 2000\space s=1800\space m$
