At first, Jason bought 2b of bookmarks and Kelvin bought 2s of stickers.
After the exchange, Jason and Kelvin each will have (b+s) bookmarks and stickers.
When Jason gave 7 bookmarks to his sister, he will be left with (b-7) bookmarks.
9(b-7) -> s
When Kelvin gave 12 stickers to his youger brother, he will be left with (s-12) stickers.
6b -> (s-12) or b -> s/6 – 2
9(s/6 – 2 – 7) -> s
1.5s – 81 -> s
s -> 162
a) At end, number of stickers Kelvin has -> 162 – 12 = 150
b) At the beginning, number of bookmarks Jason bought -> 2b -> (162/6 -2) x 2 = 50