f is an anonymous function
f = @(t,y)[-2*y(1)+y(2)+15*cos(t); 2*y(1)-y(2)];
note that your script is not correct, since t is the time vector used for the time integration and h is calculated inside the euler function. All in all
% initial value
y0 = [3,0];
% time vector
t = linspace(0,4*pi,100);
% anonymous function
f = @(t,y)[-2*y(1)+y(2)+15*cos(t); 2*y(1)-y(2)];
% solution
y = eulerfunction(f, t, y0);
% plot
figure,plot(t,y);
% your euler method
function y = eulerfunction(func, t, y0)
n = length(t);
y = nan(length(y0), n);
y(:,1) = y0(:);
for k = 1:n-1
h = t(k+1) - t(k);
y(:,k+1) = y(:,k) + h*func(t(k), y(:,k));
end
end
