The length of a rectangular painting is 3 inches longer than its width. If the diagonal is 15 inches long, what is the length of the painting?

Let the width be w inches, so the length is w + 3. By the Pythagorean theorem, w² + (w + 3)² = 15², which gives w = 9 (taking the positive solution). Therefore, the length is 9 + 3 = 12 inches, so the correct choice is 12 inches.

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