Answer
(a) 41 m/s, due east
(b) -41 m/s, due west
Work Step by Step
Let's take,
The velocity of train A relative to train B = $V_{AB}$
The velocity of train A relative to the ground = $V_{AG}$
The velocity of train B relative to the ground = $V_{BG}$
Taking east as the positive direction, we can get.
(a)$V_{AB}=V_{AG}+V_{GB}=V_{AG}-V_{BG}$ ; Let's plug known values into this equation.
$V_{AB}=13\space m/s-(-28\space m/s)=41\space m/s$ due east
(b) $V_{BA}=-V_{AB}=-41\space m/s$ due west
