Solving Systems by Addition or Subtraction

On Tuesday in class, we went over how to solve a system of equations either by addition or subtraction. We watched a video on youtube that explained all the steps to follow, and we wrote them down on paper with an example. http://www.youtube.com/watch?v=ej25myhYcSg, this is the link to the video we watched. We were assigned the four equations on the back to do for homework.

Steps:
1) Eliminate a variable by doing addition/subtraction.
2) Add the coefficients together
3) Once you eliminate one variable, solve for the other variable.
4) Substitute into the equation of your choice.
5) Solve!!

Example:
2x+3y=4
+(-2x+7y=16)
0x +10y=20
/10y /10
y=2

2x+3(2)=4
2x+6=4
-6 -6
2x=-2
x= -1


Answer: -1,2