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(-6/7)*x+1/3 = 5/21 // - 5/21
(-6/7)*x+1/3-(5/21) = 0
(-6/7)*x+1/3-5/21 = 0
2/21-6/7*x = 0 // - 2/21
-6/7*x = -2/21 // : -6/7
x = -2/21/(-6/7)
x = 1/9
x = 1/9
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