Explanation:
the case of 2 secants starting at the same point (L).
the product of the circle outer part with its total length (to the far circle arc) is constant for all secants from the same point.
so,
10(10 + 42) = 13(13 + x)
520 = 169 + 13x
351 = 13x
x = 351/13 = 27