Help me with finding the minimum of one dimensional convex function using numerical gradient method

Hi, I have to write a code for finding the minimum of one dimensional convex function using numerical gradinet method. So far I have:

 if 
    % code
 e % starts from this point
function ebo
clc
x0=2
E=1e-6
k=0.1
X1=x0
[F,g]=fun(X1)
X2=X1-sign(g)*k
[F2,g2]=fun(X2)
i=0;
 while g*g2>0
     i=i+1
     [F,g]=fun(X1);
     X2=X1-2^i*sign(g)*k; 
     X1=X2
     [F2,g2]=fun(X2);
 end
disp(X2);
disp(fun(X2));
function [F,g]=fun(x)
F=2*x^2+3;
g=4*x;

Im stuck, the gradient always shows me 0. I really need help, could you please correct mine code or help me.