1.若x>=a,则x-a<=3, a<=x<=a+3 若x<=a,则a-x<=3, a-3<=x<=a所以a-3<=x<=a+3又因为f(x)≤3的解集为{x丨-1≤x≤5}所以a=-22.因为|x+2|+|x+7|>=5,而f(x)+f(x+5)≥m对一切实数x恒成立所以m>=5