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:

Hence,

Solve for T.