Finding the absolute maximum of a 3 variable function

Question

How do I approach finding the maximum for an $f(x,y,z)$?

$f(x,y,z)$= $6.365+7335000y-24450xy-0.5* \sqrt {10614564y^4+2391210000x^2y^2-1434726000000xy^2+31858027.2zy^2+215208923890984y^2+23904276.64z^2-1244994xy+373498200y-19947.936z+166.2145}$

I need to calculate the maximum value of the function for both of these seperately.

The locus is the cylinder $0<=x<=a$, $\:y^2+z^2<= b$ where $a,b$ are parameters.

Context: This is an expression for the principal stresses in a cylindrical beam, in a project I was working. I wish to find the maximum value of stress, and the element in which it occurs. Also, I have Finite Element Analysis results to compare with. a=0.3 b=0.025.

Here is the relevant MATLAB code

    clear all
syms x;
syms y;
syms z;
syms rho;

A=[(12.73+(1467-4.89*x)*y*10000)-rho  2444.6*z-1.02+1629*y^2  2444.6*y; 2444.6*z-1.02+1629*y^2  0-rho  0; 2444.6*y  0  0-rho  ];

eqn = det(A)==0

roots = solve(eqn, rho)


r1=roots(1)
r2=roots(2)
r3=roots(3)