First all of all, I am sorry I have put a page of my text book. Somehow I need some help to understand some paragraphs in the page. If you can explain, please let me have some explanations to those in the image.
Thank you in advance.

First all of all, I am sorry I have put a page of my text book. Somehow I need some help to understand some paragraphs in the page. If you can explain, please let me have some explanations to those in the image.
Thank you in advance.

Not sure what there is to explain about 1 and 3. I will say that the second equation follows by block-matrix multiplication: $$ \pmatrix{e_1'\\e_2'} \pmatrix{x_1\\x_2} = \pmatrix{e_1'\\e_2'} (x) = \pmatrix{e_1'x\\e_2'x}= \pmatrix{y_1\\y_2} $$