How to assign a value to a variable in an equation

Hi

I have an equation and I want to assign a value to its variable

I wrote the code below but it didn't change after running the code what can I do

Thanks

My code

clc
clear all
close all
warning off
zafar_queue =readtable('zafar_queue.xlsx');
Y = zafar_queue.nVehContrib;
data = Y';
x_train =  floor(0.9*numel(data));
dataTrain =data(1:x_train);
n = length(dataTrain);
u = 0.1* randn(n,1) ;

% Import   mydata

Opt = arxOptions;
Opt.InitialCondition = 'estimate';
arx30 = @(z)ar(dataTrain,[30], Opt)
z = Y(end)
frcast = arx30(z)

The result after compiling code is above without caculating z in it