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x in (-oo:+oo)
x/21 = 1/3 // - 1/3
x/21-(1/3) = 0
x/21-1/3 = 0
1/21*x-1/3 = 0 // + 1/3
1/21*x = 1/3 // : 1/21
x = 1/3/1/21
x = 7
x = 7
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