(-1/2n^2+2m^2)÷(n-2m)
=-1/2(n^2-4m^2)÷(n-2m)
=-1/2(n+2m)(n-2m)÷(n-2m)
=-1/2(n+2m)
=-1/2 n-m
(-x^2+12xy-36y^2)÷(x-6y)
=-(x^2-12xy+36y^2)÷(x-6y)
=-(x-6y)^2÷(x-6y)
=-(x-6y)
=6y-x