Let:
y = -3x + 9 (1)
3y = -9x + 9 (2)
Replace (1) into (2)
3(-3x + 9) = -9x + 9
Use distributive property on the left hand side:
-9x + 27 = -9x + 9
Add 9x to both sides:
-9x + 27 + 9x = -9x + 9 + 9x
27 = 9
This is false, because:
27≠9
therefore, the system doesn't have solutions
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