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Math Help - Help with solving using the sustitution method

  1. #1
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    Help with solving using the sustitution method

    I really could use assistance working through this problem, as my textbook is online and I am having trouble finding the ordered pair using the substitution method.
    1) 7x + 4y= -36
    2) -2x + y = 21

    I realize this is probably simple, but I need help with the steps to solving please. I appreciate it!
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  2. #2
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    1. 7x + 4y = -36, i.e. 4y = -7x - 36.
    2. -8x + 4y = 84, i.e. 4y = 8x + 84.
    -7x - 36 = 8x + 84, i.e. -36 - 84 = 8x + 7x.
    -36 - 84 = -120 and 8x + 7x = 15x, hence 15x = -120.
    x = -120/15 = 8
    y = 2(-8) + 21 = -16 + 21 = 5
    ∴ (x, y) = (8, 5)
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  3. #3
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    Edited.
    Last edited by johnny; May 20th 2011 at 07:18 PM.
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  4. #4
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    Quote Originally Posted by johnny View Post
    1. 7x + 4y = -36, i.e. 4y = -7x - 36.
    2. -8x + 4y = 84, i.e. 4y = 8x + 84.
    -7x - 36 = 8x + 84, i.e. -36 - 84 = 8x + 7x.
    -36 - 84 = -120 and 8x + 7x = 15x, hence 15x = -120.
    x = -120/15 = 8
    y = 2(-8) + 21 = -16 + 21 = 5
    ∴ (x, y) = (8, 5)
    Thank you! You're steps are helping me understand how I reach the answer! You're answer helps me see that I got it righy! I appreciate it
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