Greg's home is 1.8 miles from his school there is a music store halfway between school and his home today Greg wants to stop at the store after school right now he is 0.36 miles away from school

I'm guessing the question is "how far does he have to walk to the store from where he is"

if he is walking home, then hmm

distance from store to school=1.8/2 since it is halfway, so 0.9 miles

he has walked 0.36 miles

so he needs to walk 0.90-0.36=0.54 miles

he needs to talk 0.54 miles to get to the store