Electrostatic force between two point charges can be calculated using Coulomb's law.
F=k_e(q_1*q_2)/r^2F=keq1⋅q2r2, where k_eke is the Coulomb's constant(k=8.99*10^9 Nm^2C^-2k=8.99⋅109Nm2C−2), q_1q1 and q_2q2 are the charges and rr is the distance.
Force between -2C−2C (at the point 4) and 5C5C (at the point 0) is
F_1=k_e(2C*5C)/4^2=5/8k_eC^2F1=ke2C⋅5C42=58keC2. The direction of F_1F1 is positive, for the two point charges have opposite signs.
Force between -3C−3C (at the point -1) and 5C5C (at the point 0) is
F_2=k_e(3C*5C)/1^2=15k_eC^2F2=ke3C⋅5C12=15keC2. This is also an attractive force, so the direction is negative.
Therefore, the net force on a charge of 5C5C at 00 is 15k_eC^2-5/8k_eC^2=115/8k_eC^215keC2−58keC2=1158keC2, and its direction is negative.