print 3D surface-fit equation

I'm trying to fit a surface model to a 3D data-set (x,y,z) using
Where z = f(x,y).
So, I'm going for the quadratic fitting with equation:

f(x,y) = ax^2+by^2+cxy+dx+ey+f
So far, I have successfully plotted the 3d-fitted-surface using
least-square method.
But, how can I be able to print/get the fitted equation of the
surface(with coefficient values) ?

I little help will be highly appreciated.
thank you.