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We knew that it was invalid ...

Hi, we have been working on the logic exercises ... ended in a 15 minute argument between two maths students and two philosophy students

The exercise is 5.6 which says:
'From P ∧ Q and ¬P, infer R'

We knew that it was invalid because of not being able to infer R when it is not involved at all in the premise,

An argument is logically valid just if there’s no possible situation in which the premises are true and the conclusion false.

however the problem arose when we were discussing the premises.

Is it a contradiction to say that ... that P and Q, then have a second statement that says it's not the case that P?

And if so, then if the premises are contradictory does that automatically make the argument invalid by default?