to find the values of x and y using substitution method you'll have:
x = -y+7 ---equation1
x-y = -9 ---equation2
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Substitute equation 1 to equation 2
x - y = -9
(-y+7) - y = -9
-y + 7 -y = -9
transpose 7 to the right side of the equation
-y - y = -9 - 7
-2y = -16
divide the whole equation by -2
y = 8
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Substitute y=8 to equation 1 to get the value of x
x = -y + 7
x = -(8) + 7
x = -8 + 7
x = -1
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therefore x=-1 and y=8
