The number of degrees in the interior angles of a hexagon is 720°.
Since the hexagon is equiangular, each interior angle is 120°.
Therefore, each exterior angle of the hexagon is 60°.
This means that each one of the exterior triangles created on the hexagon is an equilateral triangle.
The sum of the two exterior sides of each triangle is equal to twice the length of the side of the hexagon.
Since the perimeter of the hexagon is 15, the sum of the sides created on the exterior of the hexagon is twice this, or 30.