Function within a function, Heun's method, system of ODE's

I'm trying to solve the following problem:

This is the Heun's method function file I have set up:

    function [t,Y] = myHeun(fNameode, t1, t2, h)
    n = (t2-t1)/h;
    t = t1:h:t2;   
    Y=zeros(2,n+1);
    y0 = [0.1;0.1];
    Y(:,1)=y0;
    for I=1:n
       k1 = fNameode(t(I),Y(:,I)); 
       k2 = fNameode(t(I)+h,Y(:,I)+h.*k1(:,1)); 
       Y(:,I+1)= Y(:,I)+0.5*h*(k1+k2);          
    end

My problem comes when I try to program the original function. I have so far tried this:

    function z = eqn(y)
    y(1) = 0.1;
    y(2) = 0.1;
    z = [y(1) + 2*sin(3*y(2)),y(1) - 2*y(2)];
    end

I seem to have done this second file incorrectly as I am getting error messages when running the Heun's method function my calling on this second function. My question is, how to I program this function correctly and how do I use it in the Heun's method function?

Thanks.