A circle is concentric with the ellipse (x^2)/(a^2) + (y^2)/(b^2) =1 and passed through the focus F1 and F2 of the ellipse. Two curves intersect at four points. Let P be any point of intersection. If the major axis of the ellipse is 15 and the area of the triangle PF1F2 = 26 , then find the value of 4a^2 - 4b^2.