f(x)=-x^3+6x^2-m
f'(x)=-3x^2+12x
f'(x)=0
-3x^2+12x=0
x=0 x=4
f''(x)=-6x
f''(4)=-24 f(4)极大值
12=-4^3+6*4^2-m
m=20