Okay,so I read the lesson THREE times and I just can't grasp it!Help me !

Very simple, you just replace the variable with an expression.
This usually means solving for the variable. You have to pick which equation to substitute based on the easiest to solve.

So you have 2 equations:

(1) y=2x and
(2) y=2x+6

eqn #1 is the simplest, so we substitute eqn #1 into eqn #2.
so then 2x = 2x+6

Now, we can solve for x and find y by using either eqn #1 or #2 when we get the value for x.

Lol KISS, except the fact that both line do not meet, it's good :)

Ok, another example.

Take the lines y = x + 2 and y = 2x - 1

Substitution requires you to replace one of the variables (x or y) by it's equivalent.

I want to substitute the equivalent of x in the second equation.

y = x + 2
So, y - 2 = x, or x = y - 2.

I'll substitute that in the second equation; y = 2x - 1.

y = 2(y - 2) - 1

The part in brackets is what I substituted. Since x is equal to y - 2, I can replace x by y - 2 in the second equation. This way, you can solve for y.

Expanding gives y = 2y - 4 - 1, then -y = -5, y = 5.

Now that I know y, I can find x, by using the value of y.

y = x + 2
(5) = x + 2 [I can also say that I'm substituting the value of y here]
x = 5 - 2 = 3

What do all these represent? They give the point of intersection of the two lines y = x + 2 and y = 2x - 1. From my calculations, I know that they meet at the coordinate (3, -5)

:)