Is this argument correct? A Porsche is a fast car. Dan's car is not a Porsche. Therefore, Dan's car is not fast.
Let P(x) be a Porsche
Let C(x) be a fast car
Let x be Dan
$$P(x)  \rightarrow C(x) :premise$$ 
$$\neg P(x) :premise$$
$$\equiv  C(x) :False\rightarrow anything = False$$
$$or$$ $$\equiv  \neg C(x) : Modus Pollens$$
 A: We need for $C(x)$ to denote the car $x$ is a fast car.
I'm not clear what you're trying to conclude, but we cannot conclude $\lnot C$. We cannot conclude anything about how fast Dan's car only from the knowledge that his car is not a Porche.
We know Porches are fast, but other makes and models may also be fast, perhaps faster! And Dan may have a fast, "non-Porche" car.
In general: From $$P \rightarrow Q$$ $$\lnot P$$ we cannot conclude $\lnot Q$. That's a fallacy in reasoning: sometimes called "denying the antecedent". 
The error is that: 


*

*It's a  misapplication of modus ponens, which tells us that $$P\rightarrow Q$$ $$P$$ $$\therefore Q$$

*or a misapplication of modus tollens which tells us that $$P
   \rightarrow Q$$ $$\lnot Q$$ $$\therefore \lnot P$$

A: Not necessarily. Denying the consequent (modus tollens) would yield "If the car is not fast, it is not a porsche". However, you seem to be denying the antecedent, a fallacy.
For more info: http://en.wikipedia.org/wiki/Denying_the_antecedent
A: You cannot conclude that Dan's car is not fast. Dan may have a car that is fast and is not a Porsche. 
You seem confused in your symbols.
Hope that helps,
