The drawing shows a square, each side of which has a length of L = 0.250 m. Two different positive charges q1 and q2 are fixed at the corners of the square. Find the electric potential energy of a third charge q3 = -6.00 x 10-9 C placed at corner A and then at corner B.

Answer 1

I do not know what your drawing looks like but
The major thing about potentials is that you can add them as independent scalars. They are not vectors with components.
if R1 is distance from your point to q1 and R2 is distance from your point to q2
you can find k q1 q3 / R1 and k q2 q3/R2 and just add them
see http://hyperphysics.phy-astr.gsu.edu/hbase/electric/potpoi.html

Answer 2

By the way when the charges are the same sign the potential is + and if opposite it is -
so here both are - and the things will fly in from infinity if you let them go.