Question 572581
When you solve by substitution, you're looking for one of the variables to be alone on one side of one of the equations.  In this case, you have Y by itself in the 2nd equation.  What we do then is "substitute" for Y in the first equation.


So, 
4X - 4Y = 0
But we know from the 2nd equation that Y is 7X+54
So every time they use Y in the first equation, substitute this instead.
4X - 4Y = 0
4X - 4(7X+54) = 0


Multiplying to get rid of the parens:
4X - 28X - 216 = 0
-24X - 216 = 0
Add 24X to both sides:
-216 = 24X
Divide both sides by 24:
-9 = X


But we still need Y:
7X+54 = Y
7(-9)+54 = Y
-63+54=Y
-9=Y


Checking:
4(-9)-4(-9) = 0
-36 + 36 = 0 Correct
Next equation:
7(-9) + 54 = -9
-63 + 54=-9
-9 = -9   Correct!