I always teach this as trying to get rid of the same things on both sides of the equal sign. You have at least 2 cubes on both sides, so you can get rid of those. Now you have 2 triangles and 6 cubes left. All you have to do is figure out how much each triangle is worth. If 2 triangles are worth 6 cubes, 1 triangle is worth 3 cubes. If you plug it back into the problem you have 2 cubes + 3 cubes + 3 cubes = 8 cubes.