Question

Hi!

Answer 1

You can always add or subtract same value to/from both sides of equation. The same is true about multiplication and division.

In your case you have to subtract x from both sides of equation in order to get rid of it on the left side:
$x+2y-x=8-x$
$2y=8-x$

But it's easier to remember that if you want to move some member of equation to other side, you just have to change it's sign. Let's practice a bit using your equation (I have added "plus" signs where they are usually omitted to better understend what's going on):
$+x+2y=+8$

Move 2y to the right side:
$+x=+8-2y$

Move 8 to the left side:
$+x+2y-8=0$

Move both x and 2y to the right side:
$0=8-x-2y$