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 that

y = 7-x

but that's about it.

X = 7 - y

x - 7 = 7 - 7 - y

x - 7 = - y

x + 7 = y

y = x + 7