How Do I Get Flat Stomach ?

my good buddy lost 60 pounds and he looks really good and can wear tight shirts
im normal weight i went from 140lb to 134
i weigh less then my friend but i have a gut stll
my friend is taller and were both 15
i’ve been running 6 days a week for 30 min then 60 sit ups
I average 14-18 miles a week
i only eat 3 meals a day