rectangular land is divided into four rectangular lands by two straight lines, the area of three of them is 12, 18, 30 square meters respectively, what is the area of the fourth square.
一块长方形地用两条直线分成四块长方形地，其中三块长方形面积分别是12，18，30平方米，第四块面积是多少平方米．
<result>45</result>