point od division

Let the coordinates of D be (a,b).

Since AB//CD, slope of AB = slope of CD.
slope of AB = (-3-5)/(3-11) = 1
slope of CD = (9-y)/(-1-x)

Therefore,
(9-y)/(-1-x) = 1
9-y = -1-x
x-y = -10 ...(1)

Since AD//BC, slope of AD = slope of BC.
slope of AD = (-3-y)/(3-x)
slope of BC = (9-5)/(-1-11) = -1/3

Therefore,
(-3-y)/(3-x) = -1/3
-3(-3-y) = 3-x
9 + 3y = 3-x
x+3y = -6...(2)

(1)-(2):
-4y = -4
y = 1
Substitute (y=1) into (1):
x-1=-10
x=-9

Therefore, the coordinates of D are (-9,1)

If there is any mistake, please inform me.

2007-12-05 00:07:23 補充：
我計之前用電腦畫過圖, 如果用D=(9,1)畫的話, 並不能畫出一個平行四邊形(parallelogram).你可否檢查清楚題目一次? 另外你的答案是否肯定?謝謝!