I've got two Lagrange Multiplier questions I need help on.

They both say "Use Lagrange multipliers to find the maximum and minimum values of the frunction subject to the given constraint(s).

Here is the first:

$\displaystyle f(x,y)=e^{xy}; x^3+y^3=16$

I've gotten to $\displaystyle f_x=(\lambda)g_x$ which is $\displaystyle ye^{xy}=(\lambda)3x^2$, and $\displaystyle f_y=(\lambda)g_y$ which is $\displaystyle xe^{xy}=(\lambda)(-3y^2)$ and now I'm stuck... I don't know what to do.

The second is $\displaystyle f(x,y,z)=8x-4z; x^2+10y^2+z^2=5$

I've gotten to

$\displaystyle f_x=(\lambda)g_x, --> 8=(\lambda)2x$

$\displaystyle f_y=(\lambda)g_y, --> 0=(\lambda)20y$

$\displaystyle f_z=(\lambda)g_z, --> -4=(\lambda)2z$

And now I'm stuck.