Tangential speed

Hello! Usually I have my friend help me with physics questions but he is away on vacation and I don't want to bother him. Anyways, the question is:

A young boy swings a yo-yo horizontally above his head so that the yo-yo has a centripetal acceleration of 289 m/s (squared).
If the yo-yo's string is .78 m long, what is the yo-yo's tangential speed? Answer in units of m/s.

$$a=\frac{v^2}{r}$$