Hi, I'm trying to solve the following maths problem and have fallen at the first hurdle... which is funny as it relates to a maxima minima "Saddle" Point question

Find the stationary points of the surface (variables x and y) and saddle points

*f(x,y) = 3x² + 6xy + 2y³ + 12x - 24y*

Ive got it to:

i)

* dz/dx = 6x +6y +12 = 0*

ii)

*dz/dy = 6x + 6y² - 24 = 0*
I've tried substituting i into ii but all i get is a mess which doesn't make sense, im getting confused as there's three numbers that equal 0 , any ideas