hardest math subject

What do you think is the hardest math subject you have encountered in your life.

When I was in college, I failed Thermodynamics. I majored in electronics and communications engineering, Thermodynamics is one of the major subject in my course, but I think it is more applicable to Mechanical engineering. I don't quite get it. As the name implies, it is the relationship of temperature to everyday life. Hehehe.

I could have passed that subject if not with the professor I have back then. Anyways, that's a different story altogether.

One more math subject that gave me headaches in my college years was calculus, specifically, integral and differential calculus.

Now, I don't even use it in my everyday work.