Gotcha. I'm fairly new to yoga, so I don't know all the names of the poses, but one of my favorites(ie: omg, hurts so good) was one we did a couple weeks ago. You lie on your back, extend one leg out, hovering it about an inch off the ground, flex your foot so that it's flat, as though you're stepping on the back wall. At the same time, you bring your other leg up, straight as possible, and flex your foot as if you're stepping on the ceiling. You hold it for as long as you can, or as long as your yoga instructor makes you. If someone is making you hold it, guaranteed it will be burning and you'll be cursing at them under your breath, lol. You can add to the ab workout by lifting your head up while doing this. Hold is as long as possible, bring everything back down to neutral, take a moment to breathe, then switch legs.
Like I said, I do all kinds of different things, so I don't know if that would be enough on it's own to achieve a flat tummy, but it's certainly a good way to start.