结果是 - 3
由a方+3a+1=0 可知：a方+3a= - 1
3a立方+[a方+5][a方-1]-a[5a+6]
=a四次方+3a立方-a平方-6a-5
=a平方（a平方+3a-1）-6a-5
= - 2a平方- 6a-5
= - 2（a方+3a）-5
= 2-5
= - 3