If we draw a perpendicular line from one of the vertices of the triangle we get 2 right angled triangles each with altitude 9 ins and vertex angle = 30 degrees. So:-
cos 30 = 9 /h where h = one of the sides of the equilateral triangle
h = 9 / cos 30 = 10.392 inches
Therefore the perimeter of the triangle = 3 * 10.392 = 31.1769 ins