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x in (-oo:+oo)
13*x-(21*x) = 1/3 // - 1/3
13*x-(21*x)-(1/3) = 0
13*x-21*x-1/3 = 0
-8*x-1/3 = 0 // + 1/3
-8*x = 1/3 // : -8
x = 1/3/(-8)
x = -1/24
x = -1/24
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