how to do this equation x=7-y
There is nothing to do with it. If it gives you x=# or y=# then you can plug it in, but otherwise, this is the simplest way to write this equation.If you want to go further, you can figure out thaty= 7-x but that's about it.
x = 7 - yx - 7 = 7 - 7 - yx - 7 = -yx + 7 = yy = x + 7