$\begin{array}{l}a)\,\, (2x - 1)^2 - (4x^2 - 1) = 0\\ \Leftrightarrow (2x - 1)^2 - (2x - 1)(2x + 1) = 0\\ \Leftrightarrow (2x - 1)[2x - 1 - (2x + 1)] = 0\\ \Leftrightarrow (2x - 1).(-2) = 0\\ \Leftrightarrow 2x - 1 = 0\\ \Leftrightarrow x = \dfrac{1}{2}\\ b)\,\,4x^2 - 25 - (2x - 5)(2x + 7) = 0\\ \Leftrightarrow (2x - 5)(2x + 5) - (2x - 5)(2x + 7) = 0\\ \Leftrightarrow (2x - 5)[2x + 5 - (2x + 7)] = 0\\ \Leftrightarrow (2x - 5).(-2) = 0\\ \Leftrightarrow 2x - 5 = 0\\ \Leftrightarrow x = \dfrac{5}{2} \end{array}$