The drawing shows a square, each side of which has a length of $L=0.25 \mathrm{m} .$ On two corners of the square are fixed different positive charges, $q_{1}$ and $q_{2} .$ Find the electric potential energy of a third charge $q_{3}=-6.0 \times 10^{-9} \mathrm{C}$ placed at corner $A$ and then at corner $B$.