to mark X spot, the ribbon should be stretched diagonally from one end to another. Since the flag is a square, a diagonal cuts the square into 2 equal triangles. The diagonal length is the hypotenuse of the triangle, as the triangles formed are 2 right angled triangles.
Using Pythogoras' theorem, hypotenuse can be found out with the following equation;
H² = X² +X²
where H is the hypotenuse in other words the diagonal
X - length of the triangle, since its a square all lengths are equal
X- 12 inches
H² = 12² + 12²
= 144+144
H² = 288
H = 16.9 inches≈17 inches
to make the mark X he will need 2 strips of ribbons of length 17 inches each.
total length of the ribbon needed = 34 inches