Three positive point charges are located at the corners of a rectangle. find the electric field at the fourth corner if q1 = 5 nc,
q2 = 6 nc, and q3 = 7 nc. the distances are d1 = 0.6 m and d2 = 0.2 m. (let the positive x-direction be to the right, and the positive y-direction be upward.)

This would be a pretty easy superposition problem if we knew which charge was at which corner. Let's assume and you can make the adjustments to fit your diagram. I'll put q1 at the origin, q2 upper left, q3 upper right, and q4 lower left on the x-axis. Field E24 needs a distance gotten by pythag and a direction by tan-1. Value is just kq2/r(sqr). E14 is along positive x-axis again with kq1/r(sqrd). SAme with E34 in negative y direction. Add the 3 vectors and you're done.