y=3x-2,令x=0,y1=-2
y=﹣2x+4,令x=0,y2=4
令y=y,3x-2=﹣2x+4,5x=6,x3=6/5
S=1/2|y2-y1|*|x3|=1/2*(4+2)*6/5 = 18/5