It really sounds like you have a mental model of the 'social contract' concept that isn't aligned with what it means for everyone else. To be clear, it says that we (possibly tacitly/implicitly) agree to give up some freedoms to the government in exchange for their governance/defense/etc. It has nothing to do with the enforceable contracts and things you sign. It just frames the relationship between government and constituents in terms of an agreement.

So if we give up certain freedoms in this agreement (such as the 'right to not be taxed'), then it's not unjust - we have (tacitly, implicitly) consented to this. It's true that you the individual might not have consented, and might not have the means to leave if you don't like it. But the collective is consenting, and can change the constitution if they change their mind.

So taxation is not inherently unjust if the state's constitution says it can tax you by definition. You might think the social contract that we're using is bad, or that the constitution is bad, and that's fine, and you can change it democratically if enough people agree with you. But 'unjust' doesn't really work.

> if the state's constitution says it can tax you by definition

I wasn't aware that the state defines what is just and what is not. Does that mean that there are no unjust laws?

The constitution defines what it just. Laws can be unjust.

The constitution is a law. It is the supreme law, by its own definition.

Just as an exercise, do you think you can write down my counterargument? It's not hard to see what I'll probably say.

edit: here, I'll help you. For part (a) of this question, list four different meanings of the word 'law', and give an example of each. For part (b), describe what the word 'just' means when applied to each definition from part (a). (Hint: they are all different.)

You first.