A boy of mass \(m_b\) sits at one end of a seesaw, a distance \(d_1\) from the fulcrum, and a girl of mass \(m_g\) sits at the other end, a distance \(d_2\) from the fulcrum, with \(d_1 > d_2\). The seesaw is a uniform board, and it is level and balanced, with its two ends at the same height. Derive an equation for the mass \(M\) of the seesaw that keeps it balanced with the two children on it.