Urgent help with rearranging formulae

Hi people!

I have to re-arrange a formula for a current piece of coursework but i'm drawing a complete blank on what to do. I have reason to believe i know what the correct rearrangement is but i simply can't get there! :(

The formula is

s= r - √ (r² - y²)

and i have to rearrange to make y the subject.

I have found what is apparently the correct solution and it does appear to give the correct answer but i have no idea how to get there and i need to show each stage of working !

The formula which gives the right answer is

y = √(2r - s) * √(s)

Thank you to everyone who can help and look forward to becoming a part of your community, Matt