1) (9x-12x)/(6-5x)= -3x/(6-5x)
2) (4x^2-1-3x^2)/(x+1)^2=(x^2-1)/(x+1)^2=(x-1)(x+1)/(x+1)^2=(x-1)/(x+1)
3) (4b(4a-b)-5a(a-5b))/60a^2b^2=(16ab-4b^2-5a^2+25ab)/60a^2b^2=(41ab-4b^2-5a^2)/60a^2b^2
4) (m-n)(m-3n)-(m+n)(m+3n))/8n(m-n)(m+n)=(m^2-4mn+3n^2-(m^2+4mn+3n^2)/8n(m^2-n^2)=0
5) (x+1)(x^2-x+1-x^2)/(x+1)=1-x