(b) $E_{A} =$ Electric field at $M$ due to charge placed at $A$
$E_{B} =$ Electric field at $M$ due to charge placed at $B$
$E_{C} =$ Electric field at $M$ due to charge placed at $C$
As seen from figure $\left| \overrightarrow{E_{B}} \right| = \left| \overrightarrow{E_{C}} \right|,$ so net electric field at $M,\ E_{net} = E_{A};$ in the direction of vector 2