Is it permissible to eat the meat of a calf slaughtered with a knife that was not washed after being used to slaughter a pig, or to buy halal meats that have been mixed with pork, based on the premise that Allah Almighty has made the food of the People of the Book lawful?

A Muslim is not permitted to use a ritually impure knife unless it has been thoroughly purified. Whatever part of the slaughtered animal is touched by it becomes ritually impure. It is permissible to buy halal (definition) meat and permissible items from shops that also sell impermissible things. If the halal meat becomes ritually impure, it can be purified.