Question 28816
Elimination means to get rid of one of the variables so that you can solve for the other.  With the first system of equations we have 3x-y=15 and 5x+y=9.  Adding these two together we get 8x=24 (the -y and the +y added together eliminated the variable y).  Now, it is a simple matter to solve for x.


8x=24
x = 24/8 = 3


We can now go back to the original equations and by substituting 3 for x solve for y.  We can use either equation.  Let's use the first one 3x-y=15.


3x-y=15
3(3)-y=15
9-y =15
-y=15-9=6
-y=6
y = -6


I think the mistake that you made was substituting (2) instead of (3) for the value of x.  I hope this has helped you.