Four charges are placed at the corners of a square the side

Question

Solve the all parts of following question:

Question: Four charges are placed at the corners of a square. The side of the square is a = 0.30 m.

Part I: If the charge are identical and positive q = 21.0 x 10^-9 C what would be the magnitude of the force acting on each of the charges? Answer in Newton's.

Part II: We change the charges by q₁ = 21.0 x 10^-9 C, q₂ = 11.0 x 10^-9 C, q₃ = -12.0 x 10^-9 C, q₄ = -31.0 x 10^-9 C. What would be the magnitude of the force acting on charge q₃? Answer in Newton's.

Part III: We place the same charges at the corners of a rectangle of sides a and b (instead of a square). If b=2a/3 what would be the magnitude of the force acting on charge q₃? Answer in units of Newton's.

Please provide me complete solution for the above question so I can get right solution.