Oh, good. That steel trap mind of hers is working overtime, and for once, he's rather thankful he doesn't need to lay it all out for her. He can almost see the wheels turning as she puts all the details together. It's not as if the puzzle is a complicated one.

'Why', of course, is a completely unnecessary question, and his disappointment in her registers in his expression. Why else is he asking if she can manage it, after all? Because he expects her help. Obviously. Because he can't do it to himself, and Martha isn't a witch. Instead, he decides to interpret that 'why' as 'Why do it at all'.

"Narcissa Malfoy has confirmed my suspicions regarding Bellatrix's intentions." Half-true. "My options are limited, unfortunately. Martha refuses to carry a firearm, and spending my every waking moment guarding her is not an acceptable solution. If confronted by Bellatrix, she will stand very little chance on her own. This satisfies us both as a compromise: a method of communication which can not be removed, and which will allow me to Apparate directly to her at a moment's notice."

There. Very reasonable and succinct. He braces himself for an argument nevertheless.