the final line worked out on the board, r dr r, is a homophone to the phrase "hardy har har", which is basically mock laughter, an expression to mean that something is funny. so the joke would be that the answer is funny because it sounds like laughter.
So this guy is being evaluated by a psychiatrist. The psychiatrist shows him an inkblot and asks, "what does this look like to you?"
"Sex," the guy says. So the psychiatrist shows him another one. "That looks like sex too," says the guy. He shows him a third inkblot. "Yeah, that's definitely sex."
"Sir, it appears that you have a sexual fixation," says the psychiatrist.
"Well, at least I'm not the one drawing the dirty pictures."
