对原函数求导得y'=3x^2-2x-1,令y'=0,得x=1或-1/3,当x取（负无穷,-1/3)s时,y'>0,原函数单调递增；当x取（-1/3,1）时,y'<0,原函数单调递减,当x取（1,正无穷）y'>0,函数单调递增,所以x=1时函数取最小值,即1-2-1+a=1解得a=3