2x^(m+1)-8x^(m-1)
=2x^(m-1) * (x^2-4)
=2x^(m-1) * (x+2) * (x-2)
-1/4m²+1/3mn-1/9n²
= -[(m/2)^2 - 2*m/2*n/3 + (n/3)^2]
= -(m/2 - n/3)^2