Hey Ndiazndiaz2013!
This is how you do substution:
If you have an equation, for example: x - 6
And you also know that: x = 18
Then you can "substitute" the 18 for x
So then you get 18 - 6 = 12
Another example
If the question is:
y = 4
x = 3
What is 3y - 2x?
Then, because you know that y = 4, you can work out that 3y = 12.
And because you know x = 3, you can work out that 2x = 6.
12 - 6 = 6
So 3y - 2x = 6
Hope this helps!
Chen :D
