Answer
$ 7.9 \ m/s$
Work Step by Step
The minimum speed to stay at the top of the track is when the normal force equals 0, meaning that the gravitational force equals the centripetal force:
$\frac{mv^2}{r} = mg \\ v = \sqrt{gr}= \sqrt{ 9.81 \times 6.3 } = 7.9 \ m/s$
