As children grow older, their weight increases. As children grow older, they expand their vocabulary. As children's weight increases, their vocabulary expands. Statement #3 describes a correlational relationship because, although they may occur at the same time, there is no rational way in which weight gain could cause an expansion of vocabulary.

y - 4 = 0
y = 4
2x - y - 2 = 0
-y + 2x = 2
Substitute:
-4 + 2x = 2
2x = 2 + 4
2x = 6
x = 6/2
x = 3
Solution set: (3, 4)