x - 2
By division Algorithm we have,

From above relation we got that if we add -r(x) to f(x) then resulting polynomial is divisible by g(x).
Now find the remainder when f(x) is divisible by g(x),

r(x) = - x + 2
Therefore,
- r(x) = x - 2 has to be added so that resulting polynomial is divisible by g(x).