If the ground slopes downwards from the base of the post to the guy wire ground attachment, we use the Cosine Rule to find the angle between the guy wire and the sloping ground:
8.72=13.52+9.52-2(13.5)(9.5)cosα where α is the required angle.
cosα=(13.52+9.52-8.72)/256.5=0.7673 approx, making α=39.89° approx.