f'(x)=3mx^2-6(m+1)x+(3m+6)
f(x)再(0,1/2)上单调增(1/2,1)上单调减,表明x=1/2为极大值点,
因此有f'(1/2)=0
即3m/4-6(m+1)/2+3m+6=0
解得m=-4