Make a the subject

ab = 1-a

If you bring a to the other side, then you should be able to factor it out, and go from there

I got a = 1/2b Is that right?

No that doesn't seem right. How did you get to that answer?

ab = 1-a +a
2ab = 1 /b
2a=1/b /2
a=1/2b

You cannot add ab and a since they are not the same thing. What you need to do is take a factor of a out of the whole thing, like a(1+b).

I've tried it again and got a = 1-2b / 2
More like it? or still wrong?

ab = 1-a +a
2(ab) = 1
expand
2a+2b=1 -2b
2a=1-2b /2
a= 1-2b/2

hey man, what you did was completely wrong, instead of trying to do it via trial and error perhaps go to the chapter this is located and try and do the easier question
ab = 1-a
ab+a=1
a(b+1)=1
a=1/(b+1)

EDIT - you could always just drop maths