What number should replace the X to the magic square?

The old man in his mid-eighties struggles to get up from the couch then starts putting on his coat.
His wife, seeing the unexpected behavior, asks, "Where are you going?"
He replies, "I'm going to the doctor."
She says, "Why, are you sick?"
He says, "Nope, I'm going to get me some of that Viagra stuff."
Immediately the wife starts working and positioning herself to get out of her rocker and begins to put on her coat.
He says, "Where the hell are you going"?
She answers, "I'm going to the doctor, too."
He says, "Why, what do you need?"
She says, "If you're going to start using that rusty old thing, I'm getting a tetanus shot."