# Solving system of equations by addition

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• Apr 21st 2011, 06:42 AM
nasirnoor1
Solving system of equations by addition
Solve the system of equations by the addition method:
4x-3y=-28
3x+2y=-4

I know the answer is 4,-4, however, how do I figure out what number comes first when completing the answer, is it (4,-4) or (-4,4). Thanks.
• Apr 21st 2011, 06:49 AM
Quacky
The '\$\displaystyle x\$' value comes before the \$\displaystyle y\$ value usually - just think of coordinates.
• Apr 21st 2011, 12:25 PM
poirot
Is x the input and y the output or vice versa?
• Apr 21st 2011, 01:37 PM
nasirnoor1
Quote:

Originally Posted by poirot
Is x the input and y the output or vice versa?

X is the input, y is the output
• Apr 24th 2011, 10:19 AM
naikn05
4x-3y=-28
3x+2y=-4

Multiply the top by 3 and the bottom by -4

So we now get,

3(4x-3y=-28) and
-4(3x+2y=-4)

So we now have

12x - 9y = -84
-12x -8y = 16

Now if we add the 2 equations together you'll notice that the x's cancel.

So -17y = -68

Now solve for y

y = -4

Since we now know that y = -4, Plug it into either of the original given equations, I'm going to plug it into the first one for you.

4x-3y=-28 where y = -4

So we now have 4x - 3(-4) = -28

Solve for x and you'll get x = -4

So if x = -4 and y = 4

We can write it as (-4, 4)

Hope that helps boss. (: