Let R be Romo's speed relative to the water.
Let Ta be the time he spends paddling away from his starting point.
Let Tb be the time he spends paddling back towards his starting point.
It doesn't matter to the total time whether he paddles before lunch or after lunch (assuming he doesn't get a cramp), so let's just assume (mathematicians say "without loss of generalization") he travels against the current first.
Since distance equals time x speed, we can write:
28 = Ta (R-3)
28 = Tb (R+3)
and the total time is:
Ta + Tb + .5 = 6
Now you have three equations with three unknowns. Solve the first two for Ta and Tb , add them together and substitute into the third equation. Solve for R, your answer.
Hope this helps,
Stephen La Rocque.