The tangent is giving you a hint on the value of the radius.

Since the center is at (3,-1), the tangent line y=4 is the equation of the line that limits how far the point on a circle can be from its center.

So the distance from (3,-1) to (3,4) is what?

It shouldn't be much of a hassle now to get the answer you're looking for.

I hope this helps!

--Chris