Algebra: problem has negative numbers and letters like X & Y

Always follow a pattern. Question is which one. The answer comes with some practice. Here is a problem with
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  Suppose you are given two equations
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eq1) x + 2y = 9
eq2) x - y    = 8
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Use equality to change eq2. multiply it by 2

eq1) x + 2y = 9
eq2) 2x -2 y    = 16
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Use equality to combine both sides
Totaled equation:
x + 2x + 2y - 2y = 9 + 16
3x = 25
x = 25/3
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