If

v is the speed

d is the distance

t is the time

then you are supposed to know that

Let x be the distance and y be the time rowing upstream then you have

total time t = 0.8 h = 48 min

rowing upstream: y

rowing downstream 0.8 h - y

.... Multiply by the denominators:

Substitute x by 6y:

The rower needs half an hour to row upstream by a speed of 6 that means the distance upstream is 3 km.