\[b - \frac{2a}{a - b} \cdot \frac{a^{2} - b^{2}}{4a} =\]
\[= b^{\backslash 2} - \frac{2a \cdot (a - b)(a + b)}{(a - b) \cdot 4a} =\]
\[= \frac{2b - a - b}{2} = \frac{b - a}{2}\]