Coulomb's Law - Electrostatics

Hey.

I only just started this unit at school (after coming back from holidays and forgetting everything!) and I'm having trouble with the following problem:

1. The problem statement, all variables and given/known data

"Four point charges A, B, C and D are arranged on the corners of a square of side 25cm. If A and B each have a charge of +1 uC while C and D each have a charge of +2 uC, what is the resultant force on a charge of +1 uC placed at the centre of the square?"

2. Relevant equations

Coulomb's Law
f = k((Q1*Q2)/d2)

where f = force, k = 9x10^9, Q1 and Q2 are the charges in coulombs and d is the separation of the charges (m)

3. The attempt at a solution

I attempted the question, not really knowing where to begin, and got some really weird (and definitely incorrect) answers. It's probably simple and I'm just really slow .

I'd really love if someone could just outline where I start and what processes to use to get the answer so I can apply it to other problems.

Thanks so so much!

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 Calculate individual force on the charge (F_a, F_b.....), then add the total force as a vector.

 Tags coulombs law, electrostatics