let be the desired angle for rower A's velocity w/r to the river bank.

using the diagram as reference, the rower's desired horizontal displacement is meters.

rower's velocity in the horizontal direction would be ...

so ...

in the vertical direction relative to the shore, the object's distance traveled + the rower's distance traveled must sum to be meters ...

relative to the shore in the vertical direction ...

object's speed = 3 m/s

rower's speed = m/s

all that is left is to sub in the first expression for t into the above equation and solve for ... which, btw, is really easy in this particular case.