2 18*x*y - 6*x
18*x*y - 6*x^2
/ x\ 1*(x + 0)*|y - -| \ 3/
(1*(x + 0))*(y - x/3)
6*x*(-x + 3*y)
-6*x*(x - 3*y)
-6.0*x^2 + 18.0*x*y