Angular momentum of Cylinder rolling in an incline - Kleppner

Question

I was going through Kelppner's book, section 6.14 - where they calculate the angular momentum of a rolling disk (radius= b), w.r.t. an origin and without any explanation its written: "angular momentum of the center of mass of the wheel with respect to the origin:

R X MV = -MbV

Answer 1

I have a different edition of the book.

There are brackets around $\vec R\times M \vec V$ shows that what is required is the component in the $\hat z$ direction (out of the page). The cross product is in the $-\hat z$ direction (into the page).