If he have 7 bags of 15 cookies each bag, he just need to multiply the number of bags(7) by the number of cookies in each bag (14):
14×7= 98 cookies from the bags.
Then, if he already had 9 cookies on the jar, add the number of all the cookies that he bought (98) to his cookies in the Jar (9):
98+9= 107 Cookies
At the end, he will have 107 Cookies, and also, a great amount of Cookies