过B做x轴的平行线BD 叫AC于D
AC的方程于y = kx + b
-1 = 2k +b
-5= 4k + b
k = -2 b= 3
D点坐标(3,-3)
面积 = 1/2*BD*(A点-C点的纵坐标)
1/2*2*4 = 4