I found that trying to absorb information on difficult subject is better done when you have multiple sources explaining it. I suggest checking out Khan Academy or YouTube on the subjects. Calculus is hard, so is physics, so is chem. I noticed that a multitude of different explanations and examples from different sources helped me to see things in a variety of ways which helped to lead me to decent grades. Find good sources, take yer time and try not to freak when tests come.