\(E_A=\dfrac{kq}{r_A^2}=36;E_B=\dfrac{kq}{\left(r_A+AB\right)^2}=9\)
\(\Rightarrow\dfrac{E_A}{E_B}=\dfrac{\left(r_A+AB\right)^2}{r^2_A}=4\Leftrightarrow r_A+AB=2r_A\Leftrightarrow r_A=AB\)
\(E_M=\dfrac{kq}{\left(r_A+\dfrac{AB}{3}\right)^2}=\dfrac{kq}{\left(\dfrac{4}{3}r_A\right)^2}=\dfrac{9}{16}.E_A=...\left(V/m\right)\)