How to find increment function using Heun's method?

Q: For the given differential equation: dy/dt + 2yt = 4, find the increment function using Heun's method without iteration for the step from t=2 to t=4 if y(2)=3.

My code is: dydt = @(t,y) 4-2*y*t; [t,y] = heun(dydt,[2 4],3,1,[],1)

Then I got y = 3, 16, 276

The answer should be 50.00, but I don't know what I can do next to get this value.

[Jan]

If you want a suggestion to fix your heun() code, post this code.