Coulombs law and Electric Fields

HI there. I am revising for my A level Physics exam in January. I have printed out past paper questions, but have no qnswers to them! I have worked through this question and was wandering if anyone could check to see I have done it correctly.

Any help greatly appreciated.

Two point charges, each of magnitude 5x10^-6C, are placed horizontally 50mm apart at points A and B respectively.

Calculate the force exerted on a point charge of 3x10^-6C at a point P, which is 20mm from A and 30mm from B, in the cases when:

i) both charges A and B are positive (my answer 11.25N)

ii) A is positive and the charge at B is negative (my answer 2.25N)

Thank you very much for assistance!

Mike