$${\begin{align*}y_\text{mk} &= \frac{1}{M} \sum_{i=1}^3 = \frac{m_1 y_1 + m_2 y_2 + m_3 y_3}{M} \\&= \mathrm{\frac{(1,2 \ kg)(0) + (2,5 \, kg)(0) + (3,4\, kg)(120 \, cm)}{7,1 \, kg}} \\&= 58\, \mathrm{cm}\end{align*}}$$