If is to be the square of a binomial (X+Y), then X squared has to be equal to a^2 (which means X is a), and Y^2 has to be 1/4a^2 (which means Y is 1/2a).
Then, seeing that the middle term of the given expression is negative, we know the binomial we are looking for has to be .
This is verified when we square the binomial and get .