One day, david hands out flyers to 7 people. the next day, each person copies the flyer and hands them out to 7 people. the following day, these people copy their flyer and hand them out to 7 people.

how many people now have flyers?

On the first day, David hands out flyers to 7 people. So 7 people have flyers.
The Next day, each person of this 7 people hands flyers to 7 other people: So it's 7*7 = 49. So 49 people have now flyers.
The following day, each of this 49 people gives flyers to 7 people: So it's 49*7=343.

So At last, 343 people have now flyers.

Hope this Helps! :)