I am not certain what z is. If it is indeed a function, then the function needs to be described mathematically in order to do the appropriate double integration. That would either be with the integral function twice, or integral2 function once, depending on the function characteristics. Then divide by whatever A is, and take the square root of the result.
If z is just the matrix itself, I would use the trapz function twice (once down the columns, then across that row result) for the double integrals, then divide by whatever A is, then take the square root of the result.
