f(x)=-x^3+3x^2+9x+a
f'(x)=-3x^2+6x+9
=-3(x^2-2x-3)=-3(x-3)(x+1)
由此可知,
当x∈[-2,-1]时,函数单调递减,
所以
f(x)max=f(-2)=8+12-18+a=2
a=0
f(x)min=f(-1)=1+3-9=-5