Two tangents PT and QT are drawn to circle of radius 5cm from a point T located 13cm away from the centre of circle, another tangent AB is drawn to the circumference of the circle, find the length of tangent AB.

Here there is no information given  about the line AB .So data is not sufficient to find the answer as location on line AB is unknown  . 
So the question asked by you  seems to be incomplete . Please re-check your query and post again so that we can help you in a better way .