Kant argues that the test for whether a proposition is analytic is the principle of contradiction, meaning that denying the judgment produces a self-contradiction. Let’s illustrate:

“My uncle is not my relative” — how would we check whether this sentence is analytic? We see that there is a contradiction here, since it is like saying: my relative is not my relative. Now let’s try doing this with mathematics:

⁨7+5=12⁩ — let’s try Kant’s own test: 7+5/=12 — contradiction!(?)

Is that what the Rabbi means? Is my argument correct?

You’re just playing around here without adding anything. To say that something is logically necessary means that its negation is a contradiction. So what have you gained here?

Okay, but the claim that *in the real world* 5+7 oranges are 12 oranges is necessarily synthetic.
That is, given the definitions the proposition is analytic, but the fit between the definition and reality is indeed synthetic.