I know that its not the second law. I'm almost positive its the first one. Please let me know if I'm wrong. This sentence makes no sense when you put it with the third law. So, the first law is my guess...
Answer:
The value is 
Explanation:
From the question we are told that
The mass of each sphere is 
The length of the string is
The angle of with the vertical is 
The acceleration due to gravity is 
Generally the force acting between the forces is mathematically represented as

=> 
Generally from Pythagoras theorem the radius of the circular curve created by the force is

=> 
=>
=> 
=> 
=> 
Answer:
from
force =mass x acceleration
mass = force/acceleration
m = f/a
m = 7.5/15
m=0.5kg
ONEEEEEEEEEEEEEEEEEEEEEEEEE