You just said that the object is "floating".
(As soon as you said that, a picture of a duck flashed through my mind. But then I knew right away that the duck could not be an accurate representation of the situation you're describing. 340 N would be <u><em>some duck</em></u> ... about 76 pounds ... and that duck would have been caught and eaten a long time ago. I mean ... what could a 76-pound duck do ? Could it fly away ? Could it run away ? ? Not likely.)
So it's not a duck, but whatever it is, it's just sitting there on the water, floating. What's important is that it's <u><em>not accelerating</em></u> up or down. THAT tells us that the vertical forces on it are balanced so that there's NO NET vertical force on it at all.
What are the vertical forces on it ? There's gravity, pulling it DOWN with a force of 340 N, and there's buoyancy, pushing it UP. The SUM of those two forces must be <em>zero</em> ... otherwise the object would be accelerating up or down.
It's not. So (gravity) + (buoyancy) must add up to zero.
The buoyant force on the object is <em>340 N UPward.</em>
