\(F_{13}=k\cdot\dfrac{\left|q_1q_3\right|}{r_1^2}=9\cdot10^9\cdot\dfrac{\left|5\cdot10^{-8}\cdot0,25\cdot10^{-9}\right|}{0,05^2}=4,5\cdot10^{-5}N\)
\(F_{23}=k\cdot\dfrac{\left|q_2\cdot q_3\right|}{r^2_2}=9\cdot10^9\cdot\dfrac{\left|-5\cdot10^{-8}\cdot0,25\cdot10^{-9}\right|}{0,1^2}=1,125\cdot10^{-8}N\)
\(F=\left|F_{13}-F_{23}\right|=\left|4,5\cdot10^{-5}-1,125\cdot10^{-5}\right|=3,375\cdot10^{-5}N\)