Question

There are four highlights in the paragraph that show equations or phrases

Answer 1

A reflection across the line y=0 (AKA the x-axis)

and then across the line x=0 (AKA the y-axis)

is the same as a rotation of 180º

clockwise about the origin because the lines intersect at the origin and at a 90º angle.

While all of that is true, you could also be more general, because reflecting over any two lines that intersect at the origin will result in a rotation about the origin and have an angle of rotation equal to twice that of the acute angle formed between the intersecting lines.