$${\begin{align} F_\mathrm{1,res} = 2 F_{13} \cos \theta & = \mathrm{2\frac{G m_1 m_3}{a^2} \cos \theta} & \\ = \mathrm{\frac{2(6,67 \times 10^{-11}\, m^3 /kg \cdot s^2)(8,0 \, kg)(2,0\, kg)}{(0,020\, m)^2} \cos 30^\circ } & \\= \mathrm{4,6 \times 10^{-6}\, N} \end{align}}$$