What number should be added to each one of 6, 14, 18 and 38 to make them equally proportionate?

Let the number be N,
From question, (6 + N) / (14 + N) = (18 + N) / (38 + N)
⇒ (6 + N) x (38 + N ) = (18 + N ) x (14 + N )
⇒ N² + 44N + 228 = N² + 32N + 252
⇒ 12N = 24
∴ N = 2