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n in (-oo:+oo)
7*n-3*n+3/7 = -137/7 // + -137/7
7*n-3*n+3/7-(-137/7) = 0
7*n-3*n+3/7+137/7 = 0
4*n+20 = 0 // - 20
4*n = -20 // : 4
n = -20/4
n = -5
n = -5
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