There is a square with the side a and four charges $q_1=5C,q_2=2C,q_3=-2C,q_4=1C$ in upper$-$left, upper$-$right,

bottom$-$left and bottom$-$right vertex respectively. In the geometrical center of the square sketch the picture, $4$

vectors of the field generated by individual charges and the vector of the net electrostatic field.