16a² + 24ac + 9c² + 12a + 9c = (16a² + 24ac + 9c²) + (12a + 9c) =
= [(4a)² + 2 * 4a * 3c + (3c)²] + 3(4a + 3c) = (4a + 3c)² + 3(4a + 3c) =
= (4a + 3c)(4a + 3c + 3)
