(3n+x) / (m+n) - 1 = (nx) / (m-n)(m+n) |*(m-n)(m+n)
(3n+x)(m-n) - (m^2-n^2) = nx
3nm - 3n^2 + xm - xn - m^2 + n^2 = nx |-nx
3nm - 2n^2 - m^2 + x(m-2n) = 0
x(m-2n) = 2n^2+m^2 - 3nm |:(m-2n)
x = (m^2 + 2n^2-3nm) / (m-2n) = (m-n)(m-2n) / (m-2n) = m - n
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