Speed of ball changed by lengthening string

[fattydq]

A ball of mass 0.18 kg is whirled in a horizontal circle at the end of a massless string 75 cm long at a speed of 4.5 m/s. If the string is lengthened to 1.5 m while the ball is being twirled, then the speed of the ball will now be...

What law or formula applies to this question?

 

[ideasrule]

I think you're supposed to assume that external torque is 0, so the conservation law is...