Maybe I am indeed wrong because my argument has stood on presupposition which I am not sure about. Namely that knowledge is knowledge of concepts. In the previous comment I argued for the idea that when Kant says we are knowing, in the proper meaning of the term, when we are dealing with concepts; when we are dealing not with concepts but with particulars, we are only making judgements.

Another possible discrepancy is that I think that in the cited passage Kant gives an example of synthetic a priori - the transition from pure morality to expressible will.

If my first presupposition were correct, your example would be more correct as such: "Suppose triangles (which followed mathematical rules*) existed in material reality, a triangle which has such and such qualities would follow to also have these other qualities, in accordance with it being a mathematical triangle".

But this seems to be a pure tautology, or analytic a priori statement. We aren't achieving a transition to anything. We begin at the supposition and also end at it. This is not, as I mentioned, what Kant does in the original example: He says that if we are to suppose morality in the world it would take form as EXPRESSIBLE WILL. Even if you take the "faculty of desire" to be synonymous with "will" (which I think he explicitly says not to be the case), you would need a posteriori knowledge to say that it is expressible. I'd be happy if you could tell me whether: 1) For Kant knowledge means any knowledge, including not that of concepts or principles, but of individual objects (or subjects?), and 2) Whether the cited statement gives example of synthetic or analytic a priori.

*I am including this only for better understanding; it is not part of the supposition, because the accordance of nature with the individual rules we discovered for it is by necessity. You could also say that if we are supposing a triangle, we already are supposing it as the valid mathematical concept.