Draw a picture with one of the ropes. You have a right triangle with height = 10 m and angle at pole = 30 deg.
Now simply find the hypotenuse of the triangle. As the rope length will be equal to the hypotenuse.
Here is the equation: cos (30) = pole / hypotenuse
putting values we get √3 / 2 = 10 / hypotenuse
Therefore hypotenuse = 2 * 10 / √3 = 20 √3 / 3 ~ 11.55 meters