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With what velocity should a projectile be started ar the center of the Earth to reach the surface of the Earth?

When I attempt to solve this using 1/2mv^2 = -(GMm)/R I get an answer of 11.8 km/s. However, the correct answer is 7.9km/s. I know that it has something to do with the potential inside the Earth but I dont even know where to start with incorporating that. Also could someone tell me how the negative on the right hand side is cancelled. Thanks a lot!