一道GWD概率~

大家帮帮忙吧, 我无望的数学啊~~可以的话把公式, 步骤都写一下, 拜托拜托~~

In a stack of cards, 9 cards are blue and the rest are red.  If 2 cards are to be chosen at random from the stack without replacement, the probability that the cards chosen will both be blue is 6/11.  What is the number of cards in the stack?

A.   10

B.       11

C.       12

D.       15

E.    18

设总数为X，则

C（9 2）/ C（X 2）= 6/11

X=12