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x in (-oo:+oo)
21-((2/3)*x) = 63 // - 63
21-((2/3)*x)-63 = 0
(-2/3)*x-63+21 = 0
-2/3*x-42 = 0 // + 42
-2/3*x = 42 // : -2/3
x = 42/(-2/3)
x = -63
x = -63
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