There are different ways to solve this problem, but this is the way our 4th grade son tackled it. He simplified the equations by taking the same shapes off each side. The only part that was a little bit tricky was recognizing that he had two different equations that were equal to one cube.
Look back at the Seattle P-I article link below. The critique of Ingraham High's Math Chair seems a bit shakey: "students don't normally see three variables in an equation until junior year." This question required basic math reasoning ability, but no higher math concepts.