I don't know what happened for you. I have the xbox version, and this is the equation that worked for me:
(6 * 2) - 8 + 9 * 1 = 13
Spoiler: The box's prize is a random lightsaber crystal. So don't sweat it if you've got enough of those from Dantooine. There is also another puzzle--a logic puzzle--in Dxun's tomb, on the ramp in the opposite direction.