Triangle-Gmat prep

[Capthan]

For any triangle in the xy-coordinate plane, the center of T is defined to be the point whose x-coordinate in the average(mean) of the x-coordinate of the vertices of T and whose y-coordinate in the average of the y-coordinate of the vertices of T. If a certain triangle has vertices at the points(0,0) and (6,0) and center at point(3,2) what are the coordinates of the remaining vertex?
3, 4
3, 6
4, 9
6, 4
9, 6

please explain the answer. Thanks

[LoveGmat]

obviously according to the question:

3=(6+0+x)/3
2=(0+0+y)/3 so X= 3 Y=6

[rfernandez]

Perhaps not so obvious to all, but a good solution nonetheless. Well done.