Negation of a statement in two different ways. Which one is correct? What is the negation of the following statement?

All triangles are not equilateral.

Should it be :
There exists a triangle which is equilateral?
Or is it
All triangles are equilateral?
I think the former is correct. But I have seen the latter as the correct answer in two different books.
 A: I'm not a native speaker, so a comment from one (better with some degree in English language studies) would come handy, but I think that the original statement has only one reading.
Being equilateral is a property of a single triangle. All X are not P means that for all $x\in X$ it holds that $x$ does not have property $P$. That is, if $\mathcal P$ is a collection of all equilateral objects and $\mathcal T$ is a collection of triangles, the original statement formalizes as $\forall t\in \mathcal T:t\notin \mathcal P$. I think from that point it should be clear that its only negation is $\exists t\in\mathcal T:t\in \mathcal P$, as you thought yourself.
A: It is impossible to say, because the statement "All triangles are not equilateral" is ambiguous. It can mean either "No triangles are equilateral" or "Not all triangles are equilateral". You should never say in English "All A are not B".
A: What does the original statement mean?
Does it mean "For every triangle, it is true that it is not equilateral" or does it mean "It is not true that every triangle is equilateral"?
The negation of the first statement is "There exists a triangle which is equilateral".
The negation of the second statement is "Every triangle is equilateral".

Because the original statement is unclear, an exact answer to your question is impossible.
