In any triangle, the sum of all angles must equal
180o. The triangle in this case is a right triangle, with
the right angle found between the two normals (to the left and top sides
of the fiber). The path of the light inside the fiber makes the hypotenuse.
The angle between the light path and the normal to the
left side of the fiber is thus
180o - 90o - 69o = 21o.