# Systems of Equations with Substitution Method

• May 1st 2008, 03:41 PM
endlesst0m
Systems of Equations with Substitution Method
I have a test tomorrow and I can't figure out why I'm ending up with ridiculous numbers when doing this problem...

y + 3x = 7
2x + 3y = 14

Now I know that you're supposed to work with a variable with a coefficient of 1 because it helps you avoid fractions, so I figure it's best to solve for the "y" in "y + 3x = 7".

Here's what I'm doing...I'm trying to get the "y" by itself:

y + 3x = 7
-3x -3x
Is this right^? would it be y= 7 - 3x?

This is only the first step so I might need more help on this.
• May 1st 2008, 03:50 PM
icemanfan
Quote:

Originally Posted by endlesst0m
I have a test tomorrow and I can't figure out why I'm ending up with ridiculous numbers when doing this problem...

y + 3x = 7
2x + 3y = 14

Now I know that you're supposed to work with a variable with a coefficient of 1 because it helps you avoid fractions, so I figure it's best to solve for the "y" in "y + 3x = 7".

Here's what I'm doing...I'm trying to get the "y" by itself:

y + 3x = 7
-3x -3x
Is this right^? would it be y= 7 - 3x?

This is only the first step so I might need more help on this.

Yes, $y = 7 - 3x$ is correct. Using that method, you would then substitute into the second equation:

$2x + 3(7 - 3x) = 14$

$2x + 21 - 9x = 14$

$2x - 9x = -7$

$-7x = -7$

$x = 1$