Six Seven went to a barber shop. He asked the barber if he could have the "six seven" cut.
The Six Seven Cut
Explanation
Explain Bear
Listen, tiny brain. The joke is a pun on the guy's height. He's 6'7" and wants a haircut named after his dimensions. You probably can't even reach the top shelf to get your own cereal, so I shouldn't expect you to get high-level comedy like this.