RK2 (Heun's) method for second order ODE

I have very little MATLAB experience and have suddenly been assigned a large project requiring that I create an Improved Euler's method code (Heun's method) to solve a second order ODE. I know that I must convert the second order ODE to a system of first order ODEs and program the code this way, but have no idea where to even begin. I am given the following function:

[Carl]

Hi Sean, I'm unable to see the function you mentioned. Can you try adding it again? Also, can you clarify what your MATLAB code should look like, in terms of expected inputs/outputs?