Problem: what is the elimination method for this equation 3x+3y=-9, 3x-3y=21
This one is easy, because the y co-efficient in equation 1 is 3,
and the y co-efficient in equation 2 is negative 3. All we have to
do is add the equations to eliminate the y term.
3x + 3y = -9
+(3x - 3y = 21)
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6x = 12
6x = 12
x = 2
Use that in the first equation to solve for y.
3x + 3y = -9
3(2) + 3y = -9
6 + 3y = -9
6 + 3y - 6 = -9 - 6
3y = -15
y = -5
Use the second equation to check the answer.
3x - 3y = 21
3(2) - 3(-5) = 21
6 + 15 = 21
21 = 21
Answer: x = 2, y = -5