Answer:
x = 17
Explanation:
in such a constellation (two beams from the same point of origin cut through the same circle) the relative relation between the segments of these beams to the overall length of the beam have to be the same :
7 × (7+x) = 8 × (8+13)
49 + 7x = 8 × 21 = 168
7x = 119
x = 17