Question

How to do this equation x=7-y

Answer 1

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 that
y= 7-x 
but that's about it.

Answer 2

X = 7 - y
x - 7 = 7 - 7 - y
x - 7 = -y
x + 7 = y
y = x + 7