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n in (-oo:+oo)
(-6/7)*n-(2/3) = 3*n-500/21 // - 3*n-500/21
(-6/7)*n-(3*n)-(2/3)-(-500/21) = 0
(-6/7)*n-3*n-2/3+500/21 = 0
162/7-27/7*n = 0 // - 162/7
-27/7*n = -162/7 // : -27/7
n = -162/7/(-27/7)
n = 6
n = 6
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