First, let me say that you write extremely well for your age, Carissa.
Say you have these 2 equations:
2x + y = 7 [1]
x + 2y = 8 [2]
From [1], we have y = 7 - 2x ; still with me?
Now "substitute" that in [2]:
x + 2(7 - 2x) = 8
So:
x + 14 - 4x = 8
x - 4x = 8 - 14
-3x = -6
x = -6/-3
x = 2 : OK?
Now "substitute" that in one of the 2 equations; pick [1]:
2x + y = 7
2(2) + y = 7
4 + y = 7
y = 7 - 4
y = 3
...and that's solving by "substitution" ; kapish?