Ok there are a few ways you could do this, so I'll just pick one.

You have

Let's cancel out the 2pr in the denom. of the RHS by multiplying everything by 2pr.

Thus

Let's use parentheses though to show the multiplication (just to be more formal)

Follow?

Now let's rearrange some terms to get r alone. The above equation is equal to.

Now this next part might seem tricky to you. We are really one step away from isolating r. We must simply divide by and on the LHS we will just have r and on the RHS we'll get the rest of the terms. So...

Can you simplify the rest?