Answer:
10.4 inches
Explanation:
As your problem statement tells you, the face diagonal length is √72 = 6√2. The space diagonal length is computed using a second application of the Pythagorean theorem. It will be the hypotenuse of the right triangle whose legs are the face diagonal and the edge of the cube.
d² = 6² +(6√2)² = 36 +72 = 36×3
d = √(36×3) = 6√3 ≈ 10.4
The interior diagonal is about 10.4 inches long.