Extremal value of a function of two variable

Hi experts,

I wanna calculate the extremal value of a function, I have already coded the function up and have no idea how to calculate.

Could you please help me on how to calculate the extremal value of the function Z? Please see my code as below:

clc;clear;close all
syms x y
% x = 0:0.01:1;
% y = 0:0.01:1;
% [X, Y] = meshgrid(x,y);
U=@(x,y) 1.1966*1.5.*(1-x.^5.085).*(1-y.^2);
Pone(x,y)=diff(U,x)

Pone(x, y) = 

Ptwo(x,y)=diff(U,y)

Ptwo(x, y) = 

x = linspace(0,1,11);
y = linspace(0,1,11);
[X, Y] = meshgrid(x,y);
Z= U*(0.065*(Pone^2)+(Ptwo^2))^(0.5)

Z(x, y) = 

Cheers