Minimum J to change a satellite's orbit?

Question

A 40kg satellite circles the earth in an orbit with a period of 60 min. What minimum energy (in J) is required to change the orbit to another circular orbit with a period of 180 min?

I have to use: radius earth r=6.4EE6m mass earth M=6EE24kg
G=6.7EE-11 Nm^2/kg^2

I think I can assume that r becomes 3r even though an orbit isn't a circle. And the period T becomes 3T.

I know I need to use the formula for mechanical energy and also the formula for the period T^2=( 4pi^2/GM )r^3 but how do I figure out r first??
When I solve the last equation for r using T in seconds, I get 5091.24 km but this is less that the r of the earth so something is wrong.
Who can help, what am I not seeing??

Answer 1

try and put GMm/r2