$${\begin{align*}x_\text{mk} &= \frac{1}{M} \sum_{i=1}^3 = \frac{m_1 x_1 + m_2x_2 + m_3 x_3}{M} \\&= \mathrm{\frac{(1,2 \ kg)(0) + (2,5 \, kg)(140\, cm) + (3,4\, kg)(70 \, cm)}{7,1 \, kg}} \\&= 83\, \mathrm{cm}\end{align*}}$$