There is no need to find the circumference.
Make a simple sketch.
You'll find that the angle the string does with the horizontal to be:Code:/| / | / | / | 3 / | / | / | /_______| O 0.6
Then, resolving vertically, the vertical component of the tension is given by:
Since the bob is in equilibrium vertically, the resultant force = 0.
The downwards force acting on the bob is given by:
Solve for T.