1、 Premise: The truth about human knowledge lies in the position of skepticism.

? ? ? Premise: There is no way that we can know anything to be true.

No conclusion can be drawer.

The two premises are contradictory, we can't have it both ways.

2、 Premise: Each individual needs to determine what is right.

Conclusion: I think that using marijuana is ethical, so people who think smoking marijuana is unethical are wrong.

The conclusion is inconsistent with the premise. Because everyone can determine what is right, so people who think smoking marijuana is unethical are wrong perceive their stand as right.