This appears to be an error as there is (n=4) assuming you meant (n-4)
3/(n-1)=2n/(n-4)
3(n-4)=2n(n-1)
3n-12=2n^2-2n
3n-12-2n^2+2n=0
-2n^2+5n-12=0
2n^2-5n+12=0
Quadratic formula
5+-(sqrt)(-5^2-4*2*12)
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2*2
5+-(sqrt)(-71)
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4
Don't know if you just want real numbers or not as -71
5+-(sqrt)(71) i
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4