It is b. 48 because f(x) is 3
L: 1750, M:1750-480=1270
Lisa gave some stamps to Mark so Lisa now has 1750-x and Mark has 1270+x. Mark also has 3 times as many as Lisa now: (1750-x)*3=1270+x
5250-3*x=1270+x
4*x=3980, x=3980/4, x=995
So Lisa gave 995 stamps to Mark.
At first Mark had 1270 stamps. Lisa now has 1750-995=755 stamps.
Add 6 7 and two you get 15 xy subtract -11 and -4 you get positive 7xy add 15 and 7 and you get 22xy