Too X, X enough, enough X

The key point here is that if something is "too expensive", "too difficult", "too late" or whatever, then this prevents you from doing what you otherwise would.

• The car was too expensive, so I didn't buy it.
• The problem was too difficult, so I couldn't solve it.
• The party started too late, so I didn't attend.

In a sense, "enough" works the opposite way. That is, if something is "cheap enough", "easy enough", "early enough", then this enables you to do something.

• The car was cheap enough, so I bought it.
• The question was easy enough, so I posted an answer.
• The party started early enough, so I went along and stayed for a while.

Note also that when "enough" is an adverb, it follows the adjective or verb that it qualifies. When it is used as an adjective, it usually precedes the noun.

• The car had enough petrol in the tank.
• The question had enough merit to warrant being re-opened.

For many of these questions, the idea is that the word enough is not used in the same way as too or very are used; you simply don't say enough difficult, or enough expensive. When modifying an adjective, the word is enough is used after the word it modifies, not before:

He's old enough to stay out past midnight.
He's skilled enough to ski down the expert trail.

If you want to use a word before the adjective, you can use sufficiently:

He's sufficiently skilled to ski down the expert trail.

or, you can restructure the sentence, and use enough to modify a noun:

He has enough skill to ski down the expert trail.

As for this example problem:

It was ______ so we didn't get it.
a. expensive enough
b. too expensive - correct
c. enough expensive

As I mentioned previously, I can't think of an instance when I would ever say "enough expensive." As for "expensive enough", there are times I might use that wording, but not within the "It was ___, so..." construct. However, I might say:

We didn't go out to dinner after the show; the tickets to the play were expensive enough.

As for this one:

It's _____ to read; I don't understand it at all.
a. enough difficult
b. too difficult - correct
c. difficult enough

Again, (a) is just downright awkward. The third option almost works, but too difficult matches better with the speaker not understanding the reading material. If, however, the sentence had said,

It's _____ to read; I think it'll take me forever to finish that book!

then (c) might be more acceptable, because the word enough can be used for emphasis, where "difficult enough" can mean, essentially, "very difficult."

As for this one:

It's not _____ to sort things out.
a. late enough
b. too late - correct

There's nothing wrong with saying late enough, but not in this context (because it's hard to think of a context where it's too early to sort things out). Instead, one might say:

It's not late enough in the day yet to start selling our bread for half price.

(Admittedly, I'd probably opt for "too early," instead of "not late enough," but, nevertheless, that sentence is grammatical.)

I hope these explanations help you out. It's hard to tackle all six of these at once – which is why some may have voted to close the question initially. Hopefully, though, I've explained enough, so that you can figure out the ones I didn't address.

One last thought, about your comment (that this must be "the worst site for English help"): to that, I say, "Enough, already." I think there are enough helpful people on EL&U that such comments are unwarranted and unnecessary.