Answer
$x+y$
Work Step by Step
We know that since the boat is trying to travel upstream, the speed of the stream is working against the boat. For this reason, x and y, the speeds of the stream and the boat, have opposite signs. When the boat is traveling downstream, the speed of the stream actually helps the boat go faster, so both x and y should have the same signs in this case. Thus, $x+y$ is the correct expression.
