An isosceles triangle is a special triangle having at least 2 equal sides equal. But in this case, since all sides are equal having a length of 15 inches, this is actually an equilateral triangle.
When you create a perpendicular bisector from the top vertex to the midpoint of the base, you get a right triangle with the hypotenuse equal to 15 inches, a base equal to half of 15 because it was bisected, and the height. Therefore, we use the pythagorean theorem. The solution would be
h = √(15)^2 +(15/2)^
h = 13.0 meters