$${\begin{align}F_\text{12} &= \frac{G m_1 m_2}{(2a)^2} \\ & = \mathrm{\frac{(6,67 \times 10^{-11}\, m^3/kg \cdot s^2)(6,0\, kg)(4,0\, kg)}{(0,040\, m)^2}} \\ & = 4,00 \times 10^{-6} \, \mathrm{N}\end{align}}$$