Saying that line represented by equation y=x is the perpendicular bisector of line segment AB, is a fancy way of saying that point B is a mirror reflection of point A across the line y=x (in other words, the mirror is the line y=x).
This is probably beyond the scope of the GMAT, but if some point (a,b) is reflected across the line y=x, the reflected point is (b,a)
So, for example, if the point (9,2) is reflected across the line y=x, the reflected point is (2,9)
The question basically tells us that the point (-3, 3) is reflected across the line y=x, so the reflected point (point B) is at [spoiler](3, -3) [/spoiler]
Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
