cHANGING THE SUBJECT OF THE FORMULA
Make r the subject. d=2r.
I'm not sure what you mean by "make the subject."
I can only guess that you'd like to solve this equation for r, which might be what you're looking for.
By "solve this equation for r," what I mean is to rewrite it in the form "r = [some expression]."
This is also known as "isolating." By performing operations on both sides of the equation the task is achieved.
In your example we could begin by dividing both sides by 2, as demonstrated below.
d = 2r
d / 2 = (2r) / 2
d / 2 = r or reversing the equality
r = d / 2.
That didn't require many operations, but the idea is the same for more complicated equations.
Hope this helps.