∵(3x+6)²>=0
∴-(3x+6)²<=0
∴-(3x+6)²+2<=2
-(3x+6)²+2取最大值为2
∴-(3x+6)²=0
3x+6=0
x=-2
5x-[-x²-(x+2)]
=5*(-2)-[-(-2)²-(-2+2)]
=-10-[-4-0]
=-6