Question

Two equal positive charges q are placed at the opposite vertices of a square of side a. Determine and show on a diagram the electric field created by these three charges at the fourth vertex of the square.

Thanks.

Answer 1

Two charges on three vertices?
anyway
do
E = k q/r^2 in the direction from each charge to the spot and add the vectors.

Answer 2

Sorry I missed a sentence

A charge -q is placed at the third vertex