math

Solve the system by substitution.
3x-3y=-12,y=-3x-12 i see its the same problem rewritten but i confused on what i should do any help would be nice

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asked by sarah
  1. If 3x - 3y = -12, then
    x - y = -4. You also can rewrite the second equation to get
    3x + y = -12
    Therefore, adding the two equations,
    4x = -16 , so x = -4.
    That tells you that y must be 0.

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    posted by drwls
  2. @drwls: The solution you provided is correct, however, it was asked that this system be solved by means of substitution.

    In the second equation, it is given that y=-3x-12. Simply by replacing (i.e. substituting) y in the first equation by the value that was given for y in the second equation, we get:

    3x-3(-3x-12)=-12
    <=> 3x+9x+36=-12
    <=> 12x = -48
    <=> x = -4

    since y = -3x-12, y=0

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