We have that the first number of an ordered pair represents x and the second (after the comma) represents y:
(x, y) = (k, 3)
Then in this case
x = k and y = 3
We want to find k, this means that we want to find x when y = 3.
Using the given equation we find k.
In order to do that we just replace y by 3 and x by k:
x + 2y = 8
↓ replacing x = k and y = 3
k + 2· 3 = 8
↓ since 2 · 3 = 6
k + 6 = 8
↓ taking 6 to the right side of the equation
k = 8 - 6 = 2
Answer: k = 2
