Why Does My Satellite Orbit Calculation Not Match the Expected 1.8 Hour Period?

[smw1016]

At what altitude will a satellite complete a circular orbit of the Earth in 1.8 hours?

i used r=(GMT^2/4pi^2)^1/3 -Radius of the earth.

but I am keep getting wrong answer please help

 

[Redbelly98]

I think that equation is correct. You may want to double-check the units: is everything in terms of m, s, and kg? (I.e., not km for example.)