So there are 12 months in a year and he delivers 8,520 in that time. You must divide 8,520 by 12 which gives you how many he delivers in one month. 8,520/12=710. You are looking for two months so you should then multiply 710 by two giving you how much he delivers in 2 months.
710*2=1,420
Max delivers 1,420 pieces of mail in two months