This is a 2D manipulator.
The first arm rotates in the 1/2 plane, about the inertial 3 axis.
The second arm rotates about the inertial 3 axis, but at the joint:.J(2/1)
In this problem, the point of interest -- the red ball is not a "computer graphics child" of the second arm.
Rather, it is a free object: its position is computed from Equation (K) in Problem 12.2
All frame connections are shown by the combined (frame, location) notation.