For any triangle T in the xy–coordinate plan, the center of T is defined to be the point whose x–coordinate is the average (arithmetic mean) of the x–coordinates of the vertices of T and whose y–coordinate is the average of the y–coordinates of the vertices of T. If a certain triangle has vertices at the points (0,0) and (6,0) and center at the point (3,3), what are the coordinates of the remaining vertex?
Choose one:  A) (3,4)  B) (3,9)  C) (4,9)  D) (6,4)  E) (9,6)
Let (a,b) be the remaining vertex.   x: 0 + 6 + a = 3*3. Then a = 3.   y: 0 + 0 + b = 3*3.= 
<gadget id="calculator">3*3.</gadget>
<output>9</output>
9.   The answer is (3,9).
<result>B</result>
For any triangle T in the xy–coordinate plan, the center of T is defined to be the point whose x–coordinate is the average (arithmetic mean) of the x–coordinates of the vertices of T and whose y–coordinate is the average of the y–coordinates of the vertices of T. If a certain triangle has vertices at the points (0,0) and (6,0) and center at the point (3,3), what are the coordinates of the remaining vertex?