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n in (-oo:+oo)
(-2/3)*n-n+5 = 80/21 // - 80/21
(-2/3)*n-n-(80/21)+5 = 0
(-2/3)*n-n-80/21+5 = 0
25/21-5/3*n = 0 // - 25/21
-5/3*n = -25/21 // : -5/3
n = -25/21/(-5/3)
n = 5/7
n = 5/7
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