Tell your parents that your house is haunted, and that by being haunted every night, you can't sleep, you can't study, you can't do anything right. Then to make your point, spin your head around 360 degrees.
Research an Einstein equation and write that first on your note. Then say that you are failing math because you are capable of doing this kind of math, but your teachers just aren't challenging you.
Explain that you had been abducted by aliens, and were replaced by a rather stupid clone. Now that you're back and have gotten rid of the clone, you should do a lot better in math.
Or say you didn't study enough in math this semester, and that it has taught you a valuable lesson in that you get out of something, that which you put in. You will try harder next time, and to prove it, you will spend less time on the computer writing threads.