Variables are represented by letters. So in this case, our variable is d.
We want to put all the d's on one side of the equation (= sign), and all the constants (which are the numbers without a variable) on the other.
We begin with the equation
There are two options, you can either put all the variables on the left, or all the variables on the right. Whatever you do to one side of the equation, you HAVE to put it on the other side of the equation.
It is easier to find what will keep the equation positive.
So, I choose to put the variables on the left side. In order to do so, I first check if 2d is positive or negative. Since it is positive, I need to subtract it. By subtract 2d (and I have to do it to both sides), I now have 0 on the right hand side.
Now, I use the same concept for putting the constants on the right side. I want the positive 8 to become zero, so I subtract 8 from both sides.