题目

   In England, people often talk about the weather because you can experience(感受到) four seasons in one day. In the morning the weather is warm just like in spring. An hour later black clouds come, and then it rains hard. The weather gets a little cold. In the late afternoon the sky will clear(晴朗的), the sun will began to shine. It will be summer at this time of a day. In England, you can also have summer in winter, or have winter in summer. So in winter you can swim sometimes, and in summer sometimes you should take warm clothes. When you go to England, you will see that so如图，△ABC、△DCE、△GEF都是正三角形，且B、C、E、F在同一直线上，A、D、G也在同一直线上，设△ABC、△DCE、△GEF的面积分别为S1、S2、S3．当S1=4，S2=6时，S3=    ．