From
another of your threads, it appears that you are not familiar with function notation ("f(x)"), the difference between equations and expressions, the method for finding inverses, or how to work with radicals. This is material which would have been covered in high-school algebra.
You appear now to be taking a college "proofs" course. However, your degree of confusion (you do not recognize that the "Note" provided earlier is exactly the method you have been taught, and the method which is illustrated in the links provided) indicates that you have not studied the content needed to be prepared for this course. In other words, you appear to have major gaps in your background and to need intensive re-teaching.
Please consider having a serious conversation with your academic advisor, as you appear to have been improperly placed in a course beyond that for which you have currently been prepared. Unfortunately, it is not reasonably feasible to attempt here to teach the various courses of material which you need in order to understand your current material.
Either you need to back up and take courses covering the missing information, or else you need to hire a tutor who can work with you, face-to-face, to provide you with the weeks or months of missing content. And your advisor should be able to help you in this regard -- perhaps even advocating for a refund of fees paid for the inappropriate course enrollment.
My best wishes to you in your studies.