What is r in the problem 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.

Step-by-step explanation:

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.

because linear equations are a lot simpler than quadratics equations, and linear equations formula is a lot easier to memorize. that's why i think they're harder.