I took 1.5mg of Xanax (alprazolam) for anxiety about 7hrs ago. Would it be safe for me to take 15mg of hydrocodone for knee pain? Thanks

Depends. 15mg of hydrocodone is a big dose if you are not already regularly taking an opiate. You will be pretty sedated and will probably get very nauseated. The same is true for that dose of Xanax (alprazolam) but I'm assuming you are used to Xanax (alprazolam). Even if you have taken opiates regularly in the past, if its been more than 5 days your body will again be as sensitive to their effects as first time users. start with 5.
I'd wait 5 hrs more. Wow, that is a lot a Xanax (alprazolam), (alprazolam) one of the more dangerous members of the Benzodiazepine drug class, and that's a lot of Hydrocodone. Does your doctor(s) know you take this much? That can be a dangerous combination. I would wait 12 hours after Xanax (alprazolam) before taking any narcotics, and you really need to talk with your doc about alternatives.