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 Quote by berkeman Welcome to the PF! This is the problem: You cannot neglect it. You need to do the vector sum first, and then you can take the magnitude of the resultant vector.
Hi, thanks for your response. I'm not sure where I can get the 3.69 value. The resultant vector is $$4l\hat{i}+4l\hat{j}+4l\hat{k}$$.

$$F = \frac{Q^2}{4\pi\epsilon_0R^2}\cdot\hat{a}$$

Is R the magnitude of the resultant vector - it would be $$\sqrt{48l^2}$$???

I think I'm getting confused.