Dear professor:
I'm very sorry that my report disappointed you. In the first semester I selected your course and got an A, but I still not capable to learn it well. During the retaking I listened very carefully and asked my classmates about what I didn't fully understand. But the problem remains.
At the end of the semester you gave us one month for our report and it should be enough for us to finish it. But I didn't take it seriously because of some affairs of my company and family.
Now I have to prepare for a presentation next week, so please accept my apology once again.
Best regards.