Circular motion

[Link]

A satellite is orbiting the earth at an rate of 96 minutes. The radius of the earth and the mass is known. How do I calculate how high above earth the satellite is orbiting?

I tried to use formula a= 4*(3.14^2)*(r+6380)/t

But I dont have the acceleration, and If I would use newtons law of universal gravitation, I dunno where to start. Can someone help me?

[Pseudo Statistic]

T = 2xPi SquareRoot(r^3/GM)
Where G = 6.67 x 10^-11 and M is the mass of the earth, use r = radius..
You have the time, 96 x 60 for seconds..
Now, you're solving for r.