6 2 2 16*m *n - 81*x
16*m^6*n^2 - 81*x^2
-81.0*x^2 + 16.0*m^6*n^2
/ 3\ / 3\ -\9*x - 4*n*m /*\9*x + 4*n*m /
-(9*x - 4*n*m^3)*(9*x + 4*n*m^3)