Let x be the height of the pole.
The flagpole and the guy wires form a triangle with two sides of x+6.
The bottom/third side is also x+6.
So we have an equalateral triangle with sides x+6. The flagpole is perpendicular to the third side so bisects the triangle into two right triangles and the base into two equal parts of length (x+6)/2.
Since each half is a right triangle we have:
((x+6)/2)^2 + x^2 = (x+6)^2
Expand this and solve for x.