In the 2x2 example, you repeatedly reference the zero matrix notated by _2x2. Is this not the 'square' zero matrix?
If it is, then, in the equation CX=_2x2, with C (a 2x2) * X = _2x2, wouldn't X also need be a 2x2 matrix. But then it is stated that a general parameterized solution to this equation, Y, is a 2x1 matrix.
I would think that (C_2x2)(X_2x1)=_2x1.
I apologize if I am interpreting the notation incorrectly. However, if I am not, then Problems 27-30 should reference _3x1.