If the coordinate axes contain two sides of a square abcd such that a(0, a) and b(0, 0) are two vertices of the square, then point c is
(-a, a)
(0, -a)
(a, 0)
(a, a)

The answer
first of all, we need to draw a figure so that can guess easily what should be the true  answer
after that, the coordinate axes contain two sides, that means three points of the square must be on the  the coordinate axes (ABC or ABD), because A and B are already on the each axis, 
according to the question, we need to check the coordinate of C, so the point which is on the axis is A, B, C, so 
then point C is  (0, -a)

try to draw the image for more explanation