39

Number
$39$ (thirty-nine) is:


 * $3 \times 13$


 * The $15$th semiprime after $4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38$:
 * $39 = 3 \times 13$


 * The number of convex polygons that can be assembled from the complete set of $12$ hexiamonds.