Rearranging Formula

Here are 3 equations, one of which I have answered and am checking. Please could someone do a walk through as to how to answer it, with answer, because I can never seem to understand how to answer these questions and I always get them wrong.

1) Make a the subject : y=2u²+as. I put : a= √y/s - 2u (with the '/' being divided by) Corrections?

2) H = w²/2g (R²-r²) - Make r the subject.

3) T = √pbh/ 4+a² - Make a the subject.

In all cases, where I have put '/', I mean that it is structured like a fraction since I cant do that on here. Please help. I would appreciate a walk through for each with the answer which I can then learn from. Thank you.

I've attached a picture of my answers. I haven't done maths for quite a few years but I'm fairly confident.

You are an angel! Thank you! Now I can understand how it was done step by step! I can't thank you enough, and before someone says I used it for answers, no I didn't, I just can't learn unless I see someone else do it and I can use the same techniques By the way, just to let you know, you have lovely hand writing! Ahaha!