Riku did laugh, he couldn't help it when Sora was so excited about brushing his hair, but he didn't mind. He liked when Sora brushed his hair. It was kinda soothing.
"Because you can't just subtract everything. See? 2x/6 has to be made smaller first, and then you have to get ride of the 3. Its because of function, or something. Just believe me when I say this is how you do it."