x=4-y (WHAT DOSE THAT MEAN?

x=4-y

 

Think about this

3=4-1      that is tue isn't it?

if I add 1 to both sides, I get

3+1=4-1+1

and I can rewrite this as 

3+1=4

now I am going to take away 3 from both sides and I will get

3+1-3=4-3

1=4-3        that true!

 

so one step at a time I have changed

3=4-1

into

1=4-3

 

Now I will do the same thing with the letters

x=4-y

x+y=4-y+y

x+y=4

x+y-x=4-x

y=4-x

 

so I have found that if

x=4-y

then

y=4-x

 

It is more normal to have the y out the front so that is why it has been rearranged like this :)