$${\begin{align} F_{13} & = \frac{G m_1 m_2}{a^2} \\ & = \mathrm{\frac{(6,67 \times 10^{-11}\, m^3/kg \cdot s^2)(6,0\, kg)(4,0\, kg)}{(0,020\, m)^2}} \\ & = 1,00 \times 10^{-6} \, \mathrm{N} \end{align}}$$