It should be 3 15 (written in the order of up, down, left and right). The reason is that the first triangle rotates 60 degrees with its center to get the position of the second triangle number, and the second triangle also rotates 60 degrees with its center to get the position of the third triangle number. By analogy, the fourth triangle is the position where the number of the third triangle rotates 60 degrees with its center. Note that it rotates clockwise! Personal opinion, for reference only.