m/(mX-n)+n/(nX+m)=2/X
[m(nX+m)+n(mX-n)] / [(mX-n)(nX+m)]=2/X
(m^2-n^2+2mnX) / [(mn)X^2+(m^2-n^2)X-mn]=2/X
(m^2-n^2+2mnX) X=2[(mn)X^2+(m^2-n^2)X-mn]
(m^2-n^2)X-2mn=0
X=2mn/(m^2-n^2)