Answer: Hello there!
Let's define G has the number of packs of green apple, S as the number of packs of spearmint, and C as the number of packs of cinnamon.
Now we want to know the number of spearmint gum packs that Janice buys:
We know that:
G + S + C = 74
G = 2S
G - C = 6
Now we have a system of 3 equations and 3 variables, let's solve it.
In the second equation we can remove G of the equations, and replace it in the first and third equation:
G + S + C = 74
2S + S + C = 74
3S + C = 74
and
G - C = 6
2S - C = 6
Now we only have two equations, lets isolate C in the second equation
2S - C = 6
C = 2S - 6
now we can replace it in the first equation, and solve it for S
3S + C = 74
3S + 2S - 6 = 74
5S = 74 + 6 = 80
S = 80/5 = 16
She buy's 16 packs of spearmint gum.