对于有理数a、b,定义运算：a△b=3a+b/a-3b,计算（—2）△[7△（—6）]

（—2）△[7△（—6）]
=(-2)△[7*3+7/6-3*6]
=-2△25/6
=-2*25/6+25/3-25/2
=-25/3+25/3-25/2
=12.5