Frequency response simple question
They are factoring out \$e^\frac{-j\omega}{2}\$.

Note that \$e^\frac{-j\omega}{2} \cdot e^\frac{j\omega}{2} = 1\$

and \$e^\frac{-j\omega}{2} \cdot e^\frac{-j\omega}{2} = e^{-j\omega}\$