How to find Global Minima of a function with two variables?

< FIle with Data > I have a function which I obtain from a 2nd order polynomial fit using 'polyfitn'. Now I am trying to find out the global minima of this function. But I am not sure how to proceed in case of multivariabe functions. Any help? FIle

Upon applying the polyfitn on the above data, I obtain a function. How to find the global minima using this function and data? I have found out the local minima already.