How do you solve this problem? R= 1/2(r1+r2),for r2

It depends on what you mean. There are two interpretations of what you typed.

But you basically have to do a mathetical operation on both sides of the equation to get r2 alone.

For instance:
If you add something to the left, add the same thing to the right.
If you divide the left part by something, divide the right part by the same thing.
And so on.