Theme 21-4 is the fourth level of the second episode of the world Bad Piggies in the game Angry Birds.
Shoot the Yellow Bird at the pig hanging at the string to the far right. Yellow Bird's corpse should land on the tiny peice of wood that blocks the stone bricks from the pigs. The stones should topple onto the pig below. Then shoot the White Bird near the structure closest to the slingshot. Use it's ability right before you get hit, so White Bird's corpse topples the structure over. You then should get 20000 points for the birds left over, giving you three stars for beating the level.