Question

I believe there's an error in the given solution to challenge problem 2 of the potential energy unit:
The antiderivative of C/r^3 is -C/(2*r^2), not -2*C/r^2, so the potential energy is U(r)-U(infty) = -G*m_1*m_2/r + C/(2*r^2).
Then the potential energy at r_0 is -(G*m_1*m_2)^2/(2*C), which is negative, as one would expect.