A=2πr*2h

make r the subject

I can't give you the answer... It may seem too easy for some, including me. Ok, here's an example;

x + y = z

Making x the subject of formula, we get

x = z - y

as if we subtracted 'y' from both sides

Now;

'y' subject of formula gives y = z - x

Now, another one:

xy = z

Making 'y' subject of formula gives

y = z/x

as if we divided both sides by x

And finally, you get the answer you're looking for. Post back your answer to see if you got it right.