Answer:
L = 40.5 cm
Step-by-step explanation:
Since ball is rotating in horizontal plane with uniform speed
so here net force due to tension on the ball is centripetal force on the ball which is required to rotate the ball in circular path
so here we know

so here we have

as we know
m = 60 g = 0.060 kg
T = 12 N
v = 9.0 m/s
now from above formula

now we have

so L = 40.5 cm