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x in (-oo:+oo)
-((10/21)*x) = 2/3 // - 2/3
-((10/21)*x)-(2/3) = 0
(-10/21)*x-2/3 = 0
-10/21*x-2/3 = 0 // + 2/3
-10/21*x = 2/3 // : -10/21
x = 2/3/(-10/21)
x = -7/5
x = -7/5
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