I think you should use the second one.

The constant k is

I suppose you know the value of k. The only reason you can use is because th charges are of the same value.

You get r by pitagoras theorem. You put all four vectors in point c and write them analiticaly.

What you have to do here is calculate the vectors then. I'm kind of in a hurry right now, i'll try to solve it in the afternoon, with pictures and everything. Hope you can wait till then.