let point of intersection is P & center of circle is O ...
let one of the tangent touches the circle at R then
POR is a right angled triangle with OPR = 60o , POR = 90o & POR = 30o ...
now let three sides of this triangle are a,b,c then
a/sinA = b/sinB = c/sinC by using this identity we get
L/sin30 = R/sin60
L = R/root3
L is length of tangent